[angularjs] Send multipart/form-data files with angular using $http

I know there are a lot of questions about this, but I can't get this to work:

I want to upload a file from input to a server in multipart/form-data

I've tried two approaches. First:

headers: {
  'Content-Type': undefined
},

Which results in e.g. for an image

Content-Type:image/png

while it should be multipart/form-data

and the other:

headers: {
  'Content-Type': multipart/form-data
},

But this asks for a boundry header, which I believe should not be manually inserted...

What is a clean way to solve this problem? I've read that you can do

$httpProvider.defaults.headers.post['Content-Type'] = 'multipart/form-data; charset=utf-8';

But I don't want all my posts to be multipart/form-data. The default should be JSON

Here's an updated answer for Angular 4 & 5. TransformRequest and angular.identity were dropped. I've also included the ability to combine files with JSON data in one request.

Angular 5 Solution:

import {HttpClient} from '@angular/common/http';

uploadFileToUrl(files, restObj, uploadUrl): Promise<any> {
  // Note that setting a content-type header
  // for mutlipart forms breaks some built in
  // request parsers like multer in express.
  const options = {} as any; // Set any options you like
  const formData = new FormData();

  // Append files to the virtual form.
  for (const file of files) {
    formData.append(file.name, file)
  }

  // Optional, append other kev:val rest data to the form.
  Object.keys(restObj).forEach(key => {
    formData.append(key, restObj[key]);
  });

  // Send it.
  return this.httpClient.post(uploadUrl, formData, options)
    .toPromise()
    .catch((e) => {
      // handle me
    });
}

Angular 4 Solution:

// Note that these imports below are deprecated in Angular 5
import {Http, RequestOptions} from '@angular/http';

uploadFileToUrl(files, restObj, uploadUrl): Promise<any> {
  // Note that setting a content-type header
  // for mutlipart forms breaks some built in
  // request parsers like multer in express.
  const options = new RequestOptions();
  const formData = new FormData();

  // Append files to the virtual form.
  for (const file of files) {
    formData.append(file.name, file)
  }

  // Optional, append other kev:val rest data to the form.
  Object.keys(restObj).forEach(key => {
    formData.append(key, restObj[key]);
  });

  // Send it.
  return this.http.post(uploadUrl, formData, options)
    .toPromise()
    .catch((e) => {
      // handle me
    });
}

In Angular 6, you can do this:

In your service file:

 function_name(data) {
    const url = `the_URL`;
    let input = new FormData();
    input.append('url', data);   // "url" as the key and "data" as value
    return this.http.post(url, input).pipe(map((resp: any) => resp));
  }

In component.ts file: in any function say xyz,

xyz(){
this.Your_service_alias.function_name(data).subscribe(d => {   // "data" can be your file or image in base64 or other encoding
      console.log(d);
    });
}

In angular 9, Before tried 'Content-Type': undefined, but it is not worked for me then I tried the below code and It works like charms for a file object

const request = this.http.post(url, data, {
      headers: {
        'Content-Type': 'file'
      },
    });