19 Mar 2018

This article will take about 2 minutes to read.

Today I learned:

To upload files using fetch and FormData is supported in IE10+. you must not set Content-Type header.

const fileInput = document.querySelector('#your-file-input') ;
const formData = new FormData();

formData.append('file', fileInput.files[0]);

const options = {
  method: 'POST',
  body: formData,
  // If you add this, upload won't work
  // headers: {
  //   'Content-Type': 'multipart/form-data',
  // }

fetch('your-upload-url', options);

Problem I had

My API wrapper class has default content type header set to:
'Content-Type': 'application/json'

So I thought, to upload files using FormData, it would be enough to override it with:
'Content-Type': 'multipart/form-data'

But alas, it didn’t work, server couldn’t parse the files I was uploading. I’ve wasted about half an hour, and then noticed that simple HTML form was setting something else:

Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryIn312MOjBWdkffIM

It had this boundary thing I didn’t know anything about.

Then I started searching around the internet and found the solution. To set the correct boundary, I had to explicitly delete Content-Type header. In that case browser will set the correct boundary itself.

Adding this line solved it.

// Remove 'Content-Type' header to allow browser to add
// along with the correct 'boundary'
delete options.headers['Content-Type'];


What is boundary and why I had to delete the header?

Multipart form allow transfer of binary data, therefore server needs a way to know where one field’s data ends and where the next one starts.

That’s where boundary comes in. It defines a delimiter between fields we are sending in our request (similar to & for GET requests). You can define it yourself, but it is much easier to let browser do it for you.

Example payload:

Content-Disposition: form-data; name="file"; filename="my-image.jpg"
Content-Type: image/jpeg

Content-Disposition: form-data; name="field"


That’s why I had to manually delete existing header, as it didn’t contain boundary, and server was unable to parse the file correctly.

Read more about it on this StackOverflow question and MDN.


Comments (11)

26 Jun 2018, 03:28 AM

Thank you so much! Spent hours wrapping my brain around this till I read this article!

13 Jul 2018, 05:45 PM

This is what I've been looking for. Thanks!

13 Sep 2018, 07:49 PM

Super util. I tried everything until I delete it and it worked. Thanks

25 Sep 2018, 01:35 AM

Thank you so much!

so much!

so much!

so much!

so much!

so much!

so much!

I Spent more than 2 days searching for posible problem

because the post request succeed with Postman and failed when i use Angular 6 HttpClient and Django Rest Framework

then at 2:30 am i found your article!

i hope that you add more search key

for exp HttpClient of Angular and Django Rest Famework

so when some novice faces that problem they can find answers

your the best

26 Sep 2018, 07:13 AM

You made my day, thank you! :) Glad I could help.

26 Sep 2018, 08:36 PM

Thank you! This was a huge help!

16 Nov 2018, 05:33 PM

This FINALLY solved the issue I'd been having for hours, thank you so much!!!

13 Dec 2018, 06:05 AM

Please help i have trying to upload a form data with fetch and doesn't work. With postman works great but with fetch it doesnt.

13 Dec 2018, 08:57 AM

Hello Jorge,

It is really hard to help you with almost no information on the problem. Try searching for the exact error you are getting. To me, it sounds like a CORS issue, or maybe you are sending wrong headers.

Good luck!

06 Feb 2019, 11:51 PM

After spending 3 hours beating my head against this wall I found your post. Problem solved, thank you so much.

14 Feb 2019, 08:12 AM

Great solution! Thank you!!!

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