How do I Validate the File Type of a File Upload

Question

I am using  lt input type  file  id  fileUpload  runat  server  gt  to upload a file in an ASP NET application  I would like to limit the file type of the upload  example  limit to  xls or  xlsx file extensions     Both JavaScript or server-side validation are OK  as long as the server side validation would take place before the files are being uploaded - there could be some very large files uploaded  so any validation needs to take place before the actual files are uploaded

User · Answer

I agree with Chris  checking the extension is not validation of the type of file any way you look at it  Telerik s radUpload is probably your best option  it provides a ContentType property of the file being uploaded  which you can compare to known mime types  You should check for   application vnd ms-excel    application excel    application x-msexcel   and for the new 2k7 format   application vnd openxmlformatsofficedocument spreadsheetml sheet  Telerik used to sell radUpload as an individual component  but now its wrapped into the controls suite  which makes it a little more expensive  but by far its the easiest way to check for the true type

User · Answer

Ensure that you always check for the file extension in server-side to ensure that no one can upload a malicious file such as  aspx   asp etc

User · Answer

You could use a regular expression validator on the upload control      lt asp RegularExpressionValidator id  FileUpLoadValidator  runat  server  ErrorMessage  Upload Excel files only   ValidationExpression      a-zA-Z        2  w           w  w       xls  XLS  xlsx  XLSX    ControlToValidate  fileUpload  gt   lt  asp RegularExpressionValidator gt    There is also the accept attribute of the input tag    lt input type  file  accept  application msexcel  id  fileUpload  runat  server  gt    but I did not have much success when I tried this  with FF3 and IE7

User · Answer

It s pretty simple using regulare expression validator      lt asp RegularExpressionValidator id  RegularExpressionValidator1  runat  server  ErrorMessage  Only zip file is allowed   ValidationExpression        zip  ZIP    ControlToValidate  FileUpload1   gt   lt  asp RegularExpressionValidator gt    Client-Side Validation of File Types Permissible to Upload

User · Answer

Your only option seems to be client-side validation  because server side means the file was already uploaded   Also the MIME type is usually dictated by the file extension     use a JavaScript Framework like jQuery to overload the onsubmit event of the form   Then check the extension   This will limit most attempts   However if a person changes an image to extension XLS then you will have a problem   I don t know if this is an option for you  but you have more client side control when using something like Silverlight or Flash to upload   You may consider using one of these technologies for your upload process

User · Answer

Ensure that you always check for the file extension in server-side to ensure that no one can upload a malicious file such as  aspx   asp etc

User · Answer

From  javascript  you should be able to get the filename in the onsubmit handler  So in your case  you should do something like    lt form onsubmit  if  document getElementById  fileUpload   value match  xls       document getElementById  fileUpload   value match  xlsx       alert   Bad file type     return false    else   return true     gt     lt  form gt

User · Answer

As some people have mentioned  Javascript is the way to go   Bear in mind that the  validation  here is only by file extension  it won t validate that the file is a real excel spreadsheet

User · Answer

Client Side Validation Checking -  HTML    lt asp FileUpload ID  FileUpload1  runat  server    gt   lt asp Button ID  btnUpload  runat  server  Text  Upload  OnClientClick    return ValidateFile     OnClick  btnUpload Click     gt   lt br   gt   lt asp Label ID  Label1  runat  server  Text      gt    Javascript    lt script type   text javascript  gt       var validFilesTypes   bmp   gif   png   jpg   jpeg   doc   xls         function ValidateFile                 var file   document getElementById   lt   FileUpload1 ClientID  gt            var label   document getElementById   lt   Label1 ClientID  gt            var path   file value         var ext path substring path lastIndexOf      1 path length  toLowerCase           var isValidFile   false         for  var i 0  i lt validFilesTypes length  i                            if  ext  validFilesTypes i                                 isValidFile true                  break                                   if   isValidFile                          label style color  red               label innerHTML  Invalid File  Please upload a File with                   extension  n n  validFilesTypes join                              return isValidFile                  lt  script gt

User · Answer

As another respondent notes  the file type can be spoofed  e g    exe renamed  pdf   which checking for the MIME type will not prevent  i e   the  exe will show a MIME of  application pdf  if renamed as  pdf   I believe a check of the true file type can only be done server side  an easy way to check it using System IO BinaryReader is described here   http   forums asp net post 2680667 aspx  and VB version here    http   forums asp net post 2681036 aspx  Note that you ll need to know the binary  codes  for the file type s  you re checking for  but you can get them by implementing this solution and debugging the code

User · Answer

Client Side Validation Checking -  HTML    lt asp FileUpload ID  FileUpload1  runat  server    gt   lt asp Button ID  btnUpload  runat  server  Text  Upload  OnClientClick    return ValidateFile     OnClick  btnUpload Click     gt   lt br   gt   lt asp Label ID  Label1  runat  server  Text      gt    Javascript    lt script type   text javascript  gt       var validFilesTypes   bmp   gif   png   jpg   jpeg   doc   xls         function ValidateFile                 var file   document getElementById   lt   FileUpload1 ClientID  gt            var label   document getElementById   lt   Label1 ClientID  gt            var path   file value         var ext path substring path lastIndexOf      1 path length  toLowerCase           var isValidFile   false         for  var i 0  i lt validFilesTypes length  i                            if  ext  validFilesTypes i                                 isValidFile true                  break                                   if   isValidFile                          label style color  red               label innerHTML  Invalid File  Please upload a File with                   extension  n n  validFilesTypes join                              return isValidFile                  lt  script gt

User · Answer

I think there are different ways to do this  Since im not familiar with asp i can only give you some hints to check for a specific filetype   1  the safe way  get more informations about the header of the filetype you wish to pass  parse the uploaded file and compare the headers   2  the quick way  split the name of the file into two pieces -  name of the file and the ending of the file  check out the ending of the file and compare it to the filetype you want to allow to be uploaded  hope it helps

User · Answer

Well - you won t be able to do it server-side on post-back as the file will get submitted  uploaded  during the post-back   I think you may be able to do it on the client using JavaScript   Personally  I use a third party component called radUpload by Telerik   It has a good client-side and server-side API  and it provides a progress bar for big file uploads   I m sure there are open source solutions available  too

User · Answer

Avoid the standard Asp Net control and use the NeadUpload component from Brettle Development  http   www brettle com neatupload  Faster  easier to use  no worrying about the maxRequestLength parameter in config files and very easy to integrate

User · Answer

Seems like you are going to have limited options since you want the check to occur before the upload   I think the best you are going to get is to use javascript to validate the extension of the file   You could build a hash of valid extensions and then look to see if the extension of the file being uploaded existed in the hash   HTML    lt input type  file  name  FILENAME   size  20  onchange  check extension this value  upload      gt   lt input type  submit  id  upload  name  upload  value  Attach  disabled  disabled    gt    Javascript   var hash        xls     1     xlsx    1      function check extension filename submitId          var re                  var ext   filename match re         var submitEl   document getElementById submitId         if  hash ext             submitEl disabled   false          return true          else           alert  Invalid filename  please select another file            submitEl disabled   true           return false

User · Answer

I think there are different ways to do this  Since im not familiar with asp i can only give you some hints to check for a specific filetype   1  the safe way  get more informations about the header of the filetype you wish to pass  parse the uploaded file and compare the headers   2  the quick way  split the name of the file into two pieces -  name of the file and the ending of the file  check out the ending of the file and compare it to the filetype you want to allow to be uploaded  hope it helps

User · Answer

From  javascript  you should be able to get the filename in the onsubmit handler  So in your case  you should do something like    lt form onsubmit  if  document getElementById  fileUpload   value match  xls       document getElementById  fileUpload   value match  xlsx       alert   Bad file type     return false    else   return true     gt     lt  form gt

User · Answer

Avoid the standard Asp Net control and use the NeadUpload component from Brettle Development  http   www brettle com neatupload  Faster  easier to use  no worrying about the maxRequestLength parameter in config files and very easy to integrate

User · Answer

Ensure that you always check for the file extension in server-side to ensure that no one can upload a malicious file such as  aspx   asp etc

User · Answer

Your only option seems to be client-side validation  because server side means the file was already uploaded   Also the MIME type is usually dictated by the file extension     use a JavaScript Framework like jQuery to overload the onsubmit event of the form   Then check the extension   This will limit most attempts   However if a person changes an image to extension XLS then you will have a problem   I don t know if this is an option for you  but you have more client side control when using something like Silverlight or Flash to upload   You may consider using one of these technologies for your upload process

User · Answer

As some people have mentioned  Javascript is the way to go   Bear in mind that the  validation  here is only by file extension  it won t validate that the file is a real excel spreadsheet

User · Answer

Based on kd7 s reply suggesting you check for the files content type  here s a wrapper method   private bool FileIsValid FileUpload fileUpload        if   fileUpload HasFile                return false            if  fileUpload PostedFile ContentType     application vnd ms-excel             fileUpload PostedFile ContentType     application excel             fileUpload PostedFile ContentType     application x-msexcel             fileUpload PostedFile ContentType     application vnd openxmlformats-officedocument spreadsheetml sheet    this is xlsx format                   return true       return false      returning true if the file to upload is  xls or  xlsx

User · Answer

You could use a regular expression validator on the upload control      lt asp RegularExpressionValidator id  FileUpLoadValidator  runat  server  ErrorMessage  Upload Excel files only   ValidationExpression      a-zA-Z        2  w           w  w       xls  XLS  xlsx  XLSX    ControlToValidate  fileUpload  gt   lt  asp RegularExpressionValidator gt    There is also the accept attribute of the input tag    lt input type  file  accept  application msexcel  id  fileUpload  runat  server  gt    but I did not have much success when I tried this  with FF3 and IE7

User · Answer

Ensure that you always check for the file extension in server-side to ensure that no one can upload a malicious file such as  aspx   asp etc

User · Answer

Seems like you are going to have limited options since you want the check to occur before the upload   I think the best you are going to get is to use javascript to validate the extension of the file   You could build a hash of valid extensions and then look to see if the extension of the file being uploaded existed in the hash   HTML    lt input type  file  name  FILENAME   size  20  onchange  check extension this value  upload      gt   lt input type  submit  id  upload  name  upload  value  Attach  disabled  disabled    gt    Javascript   var hash        xls     1     xlsx    1      function check extension filename submitId          var re                  var ext   filename match re         var submitEl   document getElementById submitId         if  hash ext             submitEl disabled   false          return true          else           alert  Invalid filename  please select another file            submitEl disabled   true           return false

User · Answer

I think there are different ways to do this  Since im not familiar with asp i can only give you some hints to check for a specific filetype   1  the safe way  get more informations about the header of the filetype you wish to pass  parse the uploaded file and compare the headers   2  the quick way  split the name of the file into two pieces -  name of the file and the ending of the file  check out the ending of the file and compare it to the filetype you want to allow to be uploaded  hope it helps

User · Answer

As an alternative option  could you use the  accept  attribute of HTML File Input which defines which MIME types are acceptable   Definition here

User · Answer

You could use a regular expression validator on the upload control      lt asp RegularExpressionValidator id  FileUpLoadValidator  runat  server  ErrorMessage  Upload Excel files only   ValidationExpression      a-zA-Z        2  w           w  w       xls  XLS  xlsx  XLSX    ControlToValidate  fileUpload  gt   lt  asp RegularExpressionValidator gt    There is also the accept attribute of the input tag    lt input type  file  accept  application msexcel  id  fileUpload  runat  server  gt    but I did not have much success when I tried this  with FF3 and IE7

User · Answer

I agree with Chris  checking the extension is not validation of the type of file any way you look at it  Telerik s radUpload is probably your best option  it provides a ContentType property of the file being uploaded  which you can compare to known mime types  You should check for   application vnd ms-excel    application excel    application x-msexcel   and for the new 2k7 format   application vnd openxmlformatsofficedocument spreadsheetml sheet  Telerik used to sell radUpload as an individual component  but now its wrapped into the controls suite  which makes it a little more expensive  but by far its the easiest way to check for the true type

User · Answer

Avoid the standard Asp Net control and use the NeadUpload component from Brettle Development  http   www brettle com neatupload  Faster  easier to use  no worrying about the maxRequestLength parameter in config files and very easy to integrate

User · Answer

Avoid the standard Asp Net control and use the NeadUpload component from Brettle Development  http   www brettle com neatupload  Faster  easier to use  no worrying about the maxRequestLength parameter in config files and very easy to integrate

User · Answer

From  javascript  you should be able to get the filename in the onsubmit handler  So in your case  you should do something like    lt form onsubmit  if  document getElementById  fileUpload   value match  xls       document getElementById  fileUpload   value match  xlsx       alert   Bad file type     return false    else   return true     gt     lt  form gt

User · Answer

Your only option seems to be client-side validation  because server side means the file was already uploaded   Also the MIME type is usually dictated by the file extension     use a JavaScript Framework like jQuery to overload the onsubmit event of the form   Then check the extension   This will limit most attempts   However if a person changes an image to extension XLS then you will have a problem   I don t know if this is an option for you  but you have more client side control when using something like Silverlight or Flash to upload   You may consider using one of these technologies for your upload process

User · Answer

It s pretty simple using regulare expression validator      lt asp RegularExpressionValidator id  RegularExpressionValidator1  runat  server  ErrorMessage  Only zip file is allowed   ValidationExpression        zip  ZIP    ControlToValidate  FileUpload1   gt   lt  asp RegularExpressionValidator gt    Client-Side Validation of File Types Permissible to Upload

User · Answer

Well - you won t be able to do it server-side on post-back as the file will get submitted  uploaded  during the post-back   I think you may be able to do it on the client using JavaScript   Personally  I use a third party component called radUpload by Telerik   It has a good client-side and server-side API  and it provides a progress bar for big file uploads   I m sure there are open source solutions available  too

User · Answer

I agree with Chris  checking the extension is not validation of the type of file any way you look at it  Telerik s radUpload is probably your best option  it provides a ContentType property of the file being uploaded  which you can compare to known mime types  You should check for   application vnd ms-excel    application excel    application x-msexcel   and for the new 2k7 format   application vnd openxmlformatsofficedocument spreadsheetml sheet  Telerik used to sell radUpload as an individual component  but now its wrapped into the controls suite  which makes it a little more expensive  but by far its the easiest way to check for the true type

User · Answer

Well - you won t be able to do it server-side on post-back as the file will get submitted  uploaded  during the post-back   I think you may be able to do it on the client using JavaScript   Personally  I use a third party component called radUpload by Telerik   It has a good client-side and server-side API  and it provides a progress bar for big file uploads   I m sure there are open source solutions available  too

User · Answer

You could use a regular expression validator on the upload control      lt asp RegularExpressionValidator id  FileUpLoadValidator  runat  server  ErrorMessage  Upload Excel files only   ValidationExpression      a-zA-Z        2  w           w  w       xls  XLS  xlsx  XLSX    ControlToValidate  fileUpload  gt   lt  asp RegularExpressionValidator gt    There is also the accept attribute of the input tag    lt input type  file  accept  application msexcel  id  fileUpload  runat  server  gt    but I did not have much success when I tried this  with FF3 and IE7

User · Answer

Well - you won t be able to do it server-side on post-back as the file will get submitted  uploaded  during the post-back   I think you may be able to do it on the client using JavaScript   Personally  I use a third party component called radUpload by Telerik   It has a good client-side and server-side API  and it provides a progress bar for big file uploads   I m sure there are open source solutions available  too

User · Answer

Your only option seems to be client-side validation  because server side means the file was already uploaded   Also the MIME type is usually dictated by the file extension     use a JavaScript Framework like jQuery to overload the onsubmit event of the form   Then check the extension   This will limit most attempts   However if a person changes an image to extension XLS then you will have a problem   I don t know if this is an option for you  but you have more client side control when using something like Silverlight or Flash to upload   You may consider using one of these technologies for your upload process

User · Answer

As another respondent notes  the file type can be spoofed  e g    exe renamed  pdf   which checking for the MIME type will not prevent  i e   the  exe will show a MIME of  application pdf  if renamed as  pdf   I believe a check of the true file type can only be done server side  an easy way to check it using System IO BinaryReader is described here   http   forums asp net post 2680667 aspx  and VB version here    http   forums asp net post 2681036 aspx  Note that you ll need to know the binary  codes  for the file type s  you re checking for  but you can get them by implementing this solution and debugging the code

User · Answer

Seems like you are going to have limited options since you want the check to occur before the upload   I think the best you are going to get is to use javascript to validate the extension of the file   You could build a hash of valid extensions and then look to see if the extension of the file being uploaded existed in the hash   HTML    lt input type  file  name  FILENAME   size  20  onchange  check extension this value  upload      gt   lt input type  submit  id  upload  name  upload  value  Attach  disabled  disabled    gt    Javascript   var hash        xls     1     xlsx    1      function check extension filename submitId          var re                  var ext   filename match re         var submitEl   document getElementById submitId         if  hash ext             submitEl disabled   false          return true          else           alert  Invalid filename  please select another file            submitEl disabled   true           return false

User · Answer

I agree with Chris  checking the extension is not validation of the type of file any way you look at it  Telerik s radUpload is probably your best option  it provides a ContentType property of the file being uploaded  which you can compare to known mime types  You should check for   application vnd ms-excel    application excel    application x-msexcel   and for the new 2k7 format   application vnd openxmlformatsofficedocument spreadsheetml sheet  Telerik used to sell radUpload as an individual component  but now its wrapped into the controls suite  which makes it a little more expensive  but by far its the easiest way to check for the true type

User · Answer

Based on kd7 s reply suggesting you check for the files content type  here s a wrapper method   private bool FileIsValid FileUpload fileUpload        if   fileUpload HasFile                return false            if  fileUpload PostedFile ContentType     application vnd ms-excel             fileUpload PostedFile ContentType     application excel             fileUpload PostedFile ContentType     application x-msexcel             fileUpload PostedFile ContentType     application vnd openxmlformats-officedocument spreadsheetml sheet    this is xlsx format                   return true       return false      returning true if the file to upload is  xls or  xlsx

User · Answer

I think there are different ways to do this  Since im not familiar with asp i can only give you some hints to check for a specific filetype   1  the safe way  get more informations about the header of the filetype you wish to pass  parse the uploaded file and compare the headers   2  the quick way  split the name of the file into two pieces -  name of the file and the ending of the file  check out the ending of the file and compare it to the filetype you want to allow to be uploaded  hope it helps

User · Answer

Seems like you are going to have limited options since you want the check to occur before the upload   I think the best you are going to get is to use javascript to validate the extension of the file   You could build a hash of valid extensions and then look to see if the extension of the file being uploaded existed in the hash   HTML    lt input type  file  name  FILENAME   size  20  onchange  check extension this value  upload      gt   lt input type  submit  id  upload  name  upload  value  Attach  disabled  disabled    gt    Javascript   var hash        xls     1     xlsx    1      function check extension filename submitId          var re                  var ext   filename match re         var submitEl   document getElementById submitId         if  hash ext             submitEl disabled   false          return true          else           alert  Invalid filename  please select another file            submitEl disabled   true           return false

User · Answer

As an alternative option  could you use the  accept  attribute of HTML File Input which defines which MIME types are acceptable   Definition here

User · Answer

As some people have mentioned  Javascript is the way to go   Bear in mind that the  validation  here is only by file extension  it won t validate that the file is a real excel spreadsheet

User · Answer

As some people have mentioned  Javascript is the way to go   Bear in mind that the  validation  here is only by file extension  it won t validate that the file is a real excel spreadsheet

User · Answer

From  javascript  you should be able to get the filename in the onsubmit handler  So in your case  you should do something like    lt form onsubmit  if  document getElementById  fileUpload   value match  xls       document getElementById  fileUpload   value match  xlsx       alert   Bad file type     return false    else   return true     gt     lt  form gt

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