Example of multipart/form-data

Question

I am wondering if anyone can share with me an example of multipart/form-data that contains:

Some form parameters
Multiple files

possible duplicate of [What should a Multipart HTTP request with multiple files look like?](http://stackoverflow.com/questions/913626/what-should-a-multipart-http-request-with-multiple-files-look-like) — Ciro Santilli OurBigBook.com, May 07 '14 at 12:41
Go here: http://www.w3.org/TR/html401/interact/forms.html#h-17.13.4 In `17.13.4 Form content types` you'll find what you seek. — Andrew Barber, Nov 21 '10 at 16:14
Multipart upload uploads large files by pieces. Multifile upload uploads many small files. What are you asking about? — Gangnus, Jun 17 '16 at 06:39

score 168 · Answer 1 · edited Dec 27 '22 at 00:26

EDIT: I am maintaining a similar, but more in-depth answer at: https://stackoverflow.com/a/28380690/895245

To see exactly what is happening, use nc -l or an ECHO server and a user agent like a browser or cURL.

Save the form to an .html file:

<form action="http://localhost:8000" method="post" enctype="multipart/form-data">
  <p><input type="text" name="text" value="text default">
  <p><input type="file" name="file1">
  <p><input type="file" name="file2">
  <p><button type="submit">Submit</button>
</form>

Create files to upload:

echo 'Content of a.txt.' > a.txt
echo '<!DOCTYPE html><title>Content of a.html.</title>' > a.html

Run:

nc -l localhost 8000

Open the HTML on your browser, select the files and click on submit and check the terminal.

nc prints the request received. Firefox sent:

POST / HTTP/1.1
Host: localhost:8000
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:29.0) Gecko/20100101 Firefox/29.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Cookie: __atuvc=34%7C7; permanent=0; _gitlab_session=226ad8a0be43681acf38c2fab9497240; __profilin=p%3Dt; request_method=GET
Connection: keep-alive
Content-Type: multipart/form-data; boundary=---------------------------9051914041544843365972754266
Content-Length: 554

-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name="text"

text default
-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name="file1"; filename="a.txt"
Content-Type: text/plain

Content of a.txt.

-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name="file2"; filename="a.html"
Content-Type: text/html

<!DOCTYPE html><title>Content of a.html.</title>

-----------------------------9051914041544843365972754266--

Aternativelly, cURL should send the same POST request as your a browser form:

nc -l localhost 8000
curl -F "text=default" -F "file1=@a.html" -F "file1=@a.txt" localhost:8000

You can do multiple tests with:

while true; do printf '' | nc -l localhost 8000; done

Obnoxious and non-immediately-evident stuff: `boundary=---------------------------9051914041544843365972754266` is *two hyphens shorter* then the actual boundaries in the data. This is really, **really** hard to see with all the hyphens strung together. — Fake Name, Jun 02 '17 at 06:39
`curl --trace-ascii ..... ` is also handy for viewing sent and received data. — Craig Hicks, Jul 08 '18 at 20:09
`curl -trace ....` will also show binary. Handy to observe vs . — Craig Hicks, Jul 08 '18 at 20:32
@FakeName - That boundary was automatically created by curl. — Craig Hicks, Jul 08 '18 at 21:46
boundary is always -- shorter. Each MIME section separator (boundary) contains two extra dashes in the front and the trailing boundary separator contains four extra dashes: two in the front and two at the end. — Sergey Kuznetsov, Sep 23 '18 at 01:56

score 39 · Answer 2 · edited Dec 13 '17 at 09:05

Many thanks to @Ciro Santilli answer! I found that his choice for boundary is quite "unhappy" because all of thoose hyphens: in fact, as @Fake Name commented, when you are using your boundary inside request it comes with two more hyphens on front:

Example:

POST / HTTP/1.1
HOST: host.example.com
Cookie: some_cookies...
Connection: Keep-Alive
Content-Type: multipart/form-data; boundary=12345

--12345
Content-Disposition: form-data; name="sometext"

some text that you wrote in your html form ...
--12345
Content-Disposition: form-data; name="name_of_post_request" filename="filename.xyz"

content of filename.xyz that you upload in your form with input[type=file]
--12345
Content-Disposition: form-data; name="image" filename="picture_of_sunset.jpg"

content of picture_of_sunset.jpg ...
--12345--

I found on this w3.org page that is possible to incapsulate multipart/mixed header in a multipart/form-data, simply choosing another boundary string inside multipart/mixed and using that one to incapsulate data. At the end, you must "close" all boundary used in FILO order to close the POST request (like:

POST / HTTP/1.1
...
Content-Type: multipart/form-data; boundary=12345

--12345
Content-Disposition: form-data; name="sometext"

some text sent via post...
--12345
Content-Disposition: form-data; name="files"
Content-Type: multipart/mixed; boundary=abcde

--abcde
Content-Disposition: file; file="picture.jpg"

content of jpg...
--abcde
Content-Disposition: file; file="test.py"

content of test.py file ....
--abcde--
--12345--

Take a look at the link above.

Why you don't separate all properties in `Content-Disposition` with `;`? — kelin, Jan 24 '18 at 23:26

score 2 · Answer 3 · edited Sep 27 '22 at 21:44

There is an example of the multipart data (Angular):

trip-upload.component.html

<form [formGroup]="form" enctype="multipart/form-data" (ngSubmit)="submitForm()">
  <div class="form-group">
    <label for="trip">Trip:</label>
    <input formControlName="name"  type="text" id="trip" name="trip" placeholder="Name of the trip">
  </div>

  <div class="form-group">
    <label for="guide">Guide for the trip:</label>
    <input formControlName="guide" type="file" id="guide" name="guide" (change)="uploadFile($event,'guide')">
  </div>

  <div class="form-group">
    <label for="photo">Guide for the trip:</label>
    <input formControlName="photo" type="image" id="photo" name="photo" (change)="uploadFile($event, 'photo')">
  </div>

  <div class="form-group">
    <button class="btn">Upload files</button>
  </div>
</form>

2.trip-upload.component.ts

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup } from "@angular/forms";
import { HttpClient } from '@angular/common/http';
    
@Component({
  selector: 'trip-upload',
  templateUrl: './trip-upload.component.html',
  styleUrls: ['./trip-upload.component.css']
})
    
export class TripUploadComponent implements OnInit {
  public form: FormGroup;
    
  constructor(public fb: FormBuilder, private http: HttpClient) {}
    
  ngOnInit() {
    this.form = this.fb.group({
      name: [''],
      photo: [null],
      guide: [null]
    })
  }
    
  uploadFile(event, fileType: string) {
    this.updateFileFormControl(event, fileType);
  }
    
  submitForm() {
    let formData: any = newFormData();
    Object.keys(this.form.controls).forEach(formControlName => {
      formData.append(formControlName, this.form.get(formControlName).value);
    });
    
    this.http.post('http://localhost:4200/api/trip', formData).subscribe(
      (response) =>console.log(response),
      (error) =>console.log(error)
    )
  }
    
  private updateFileFormControl(event: Event, formControlName: string) {
    const file = (event.target as HTMLInputElement).files[0];
    this.form.controls[formControlName].patchValue([file]);
    this.form.get(formControlName).updateValueAndValidity()
  }
}

Multipart response

When Browser understand which enctype you use in your form for HTTP POST requests, user-agent configure 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:

Example of multipart/form-data

3 Answers3

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