application x-www-form-urlencoded or multipart form-data

Question

In HTTP there are two ways to POST data  application x-www-form-urlencoded and multipart form-data  I understand that most browsers are only able to upload files if multipart form-data is used  Is there any additional guidance when to use one of the encoding types in an API context  no browser involved   This might e g  be based on    data size existence of non-ASCII characters existence on  unencoded  binary data the need to transfer additional data  like filename    I basically found no formal guidance on the web regarding the use of the different content-types so far

User · Answer

I agree with much that Manuel has said  In fact  his comments refer to this url     http   www w3 org TR html401 interact forms html h-17 13 4      which states      The content type    application x-www-form-urlencoded  is   inefficient for sending large   quantities of binary data or text   containing non-ASCII characters  The   content type  multipart form-data    should be used for submitting forms   that contain files  non-ASCII data    and binary data    However  for me it would come down to tool framework support     What tools and frameworks do you expect your API users to be building their apps with    Do they have frameworks or components they can use that favour one method over the other    If you get a clear idea of your users  and how they ll make use of your API  then that will help you decide  If you make the upload of files hard for your API users then they ll move away  of you ll spend a lot of time on supporting them   Secondary to this would be the tool support YOU have for writing your API and how easy it is for your to accommodate one upload mechanism over the other

User · Answer

TL DR  Summary  if you have binary  non-alphanumeric  data  or a significantly sized payload  to transmit  use multipart form-data  Otherwise  use application x-www-form-urlencoded     The MIME types you mention are the two Content-Type headers for HTTP POST requests that user-agents  browsers  must support   The purpose of both of those types of requests is to send a list of name value pairs to the server   Depending on the type and amount of data being transmitted  one of the methods will be more efficient than the other   To understand why  you have to look at what each is doing under the covers   For application x-www-form-urlencoded  the body of the HTTP message sent to the server is essentially one giant query string -- name value pairs are separated by the ampersand   amp    and names are separated from values by the equals symbol       An example of this would be  nbsp   MyVariableOne ValueOne amp MyVariableTwo ValueTwo  According to the specification       Reserved and  non-alphanumeric characters are replaced by   HH   a percent sign and two hexadecimal digits representing the ASCII code of the character   That means that for each non-alphanumeric byte that exists in one of our values  it s going to take three bytes to represent it   For large binary files  tripling the payload is going to be highly inefficient   That s where multipart form-data comes in   With this method of transmitting name value pairs  each pair is represented as a  part  in a MIME message  as described by other answers    Parts are separated by a particular string boundary  chosen specifically so that this boundary string does not occur in any of the  value  payloads    Each part has its own set of MIME headers like Content-Type  and particularly Content-Disposition  which can give each part its  name    The value piece of each name value pair is the payload of each part of the MIME message   The MIME spec gives us more options when representing the value payload -- we can choose a more efficient encoding of binary data to save bandwidth  e g  base 64 or even raw binary    Why not use multipart form-data all the time   For short alphanumeric values  like most web forms   the overhead of adding all of the MIME headers is going to significantly outweigh any savings from more efficient binary encoding

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Just a little hint from my side for uploading HTML5 canvas image data   I am working on a project for a print-shop and had some problems due to uploading images to the server that came from an HTML5 canvas element  I was struggling for at least an hour and I did not get it to save the image correctly on my server   Once I set the  contentType option of my jQuery ajax call to application x-www-form-urlencoded everything went the right way and the base64-encoded data was interpreted correctly and successfully saved as an image     Maybe that helps someone

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I don t think HTTP is limited to POST in multipart or x-www-form-urlencoded  The Content-Type Header is orthogonal to the HTTP POST method  you can fill MIME type which suits you   This is also the case for typical HTML representation based webapps  e g  json payload became very popular for transmitting payload for ajax requests   Regarding Restful API over HTTP the most popular content-types I came in touch with are application xml and application json  application xml   data-size  XML very verbose  but usually not an issue when using compression and thinking that the write access case  e g  through POST or PUT  is much more rare as read-access  in many cases it is  lt 3  of all traffic   Rarely there where cases where I had to optimize the write performance existence of non-ascii chars  you can use utf-8 as encoding in XML existence of binary data  would need to use base64 encoding filename data  you can encapsulate this inside field in XML  application json  data-size  more compact less that XML  still text  but you can compress non-ascii chars  json is utf-8 binary data  base64  also see json-binary-question  filename data  encapsulate as own field-section inside json  binary data as own resource I would try to represent binary data as own asset resource  It adds another call but decouples stuff better  Example images   POST  images Content-type  multipart mixed  boundary  xxxx       multipart data  201 Created Location  http   imageserver org    foo jpg    In later resources you could simply inline the binary resource as link    lt main-resource gt        lt link href  http   imageserver org    foo jpg   gt   lt  main-resource gt

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READ AT LEAST THE FIRST PARA HERE   I know this is 3 years too late  but Matt s  accepted  answer is incomplete and will eventually get you into trouble  The key here is that  if you choose to use multipart form-data  the boundary must not appear in the file data that the server eventually receives   This is not a problem for application x-www-form-urlencoded  because there is no boundary  x-www-form-urlencoded can also always handle binary data  by the simple expedient of turning one arbitrary byte into three 7BIT bytes  Inefficient  but it works  and note that the comment about not being able to send filenames as well as binary data is incorrect  you just send it as another key value pair    The problem with multipart form-data is that the boundary separator must not be present in the file data  see RFC 2388  section 5 2 also includes a rather lame excuse for not having a proper aggregate MIME type that avoids this problem    So  at first sight  multipart form-data is of no value whatsoever in any file upload  binary or otherwise  If you don t choose your boundary correctly  then you will eventually have a problem  whether you re sending plain text or raw binary - the server will find a boundary in the wrong place  and your file will be truncated  or the POST will fail   The key is to choose an encoding and a boundary such that your selected boundary characters cannot appear in the encoded output  One simple solution is to use base64  do not use raw binary   In base64 3 arbitrary bytes are encoded into four 7-bit characters  where the output character set is  A-Za-z0-9      i e  alphanumerics           or         is a special case  and may only appear at the end of the encoded output  as a single   or a double     Now  choose your boundary as a 7-bit ASCII string which cannot appear in base64 output  Many choices you see on the net fail this test - the MDN forms docs  for example  use  blob  as a boundary when sending binary data - not good  However  something like   blob   will never appear in base64 output

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If you need to use Content-Type x-www-urlencoded-form then DO NOT use FormDataCollection as parameter  In asp net Core 2  FormDataCollection has no default constructors which is required by Formatters  Use IFormCollection instead    public IActionResult Search  FromForm IFormCollection type                return Ok

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