Uploading files using 'fetch' and 'FormData'

Today I learned:

To upload files using fetch and FormData (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 (45)

26. Jun 2018, 03:28

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

13. Jul 2018, 17:45

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

13. Sep 2018, 19:49

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

25. Sep 2018, 01:35

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

you're the best

26. Sep 2018, 07:13

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

26. Sep 2018, 20:36

Thank you! This was a huge help!

16. Nov 2018, 17:33

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

13. Dec 2018, 06:05

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

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, 23:51

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

Great solution! Thank you!!!

11. Mar 2019, 10:39

It doesn't work for me. When I try to delete the content-type field, it always give me error, it seems the field is not even defined.


delete options.headers['Content-Type'];


Uncaught TypeError: Cannot convert undefined or null to object
11. Mar 2019, 11:01

Hello Chap,

That means options.headers is null or undefined. You probably have to check if it is defined before trying to delete it's property.

Something like:

if (options && options.headers) {
delete options.headers['Content-Type'];


Thankful User
14. Mar 2019, 12:54

I would have never been able to find out about this on my own, you've saved me hours of searching, thank you !!

04. Apr 2019, 06:53

hey, where exactly should we put this?

15. May 2019, 20:50

Thanks so much. I have been on this for close to 7 hours figuring out why it's not working

04. Jun 2019, 19:39

delete options.headers['Content-Type'];

where do we put this?

04. Jun 2019, 20:25

There is a code sample in the first part of the post. You just need to remove "Content-Type" from options that you pass to fetch. Hope that helps!

05. Jun 2019, 14:23

Hi, my upload is working, the issue i am facing is my backend is sending a map as response after this upload but the fetch response.ok always come false. please help here. struck in this from long time.

11. Jul 2019, 07:55

I love you, my dude

19. Jul 2019, 17:48

I'm quite new to Express. Could you elaborate where this code goes "delete options.headers[‘Content-Type’];". Thanks

21. Sep 2019, 22:40

Just saved my life

Serkan Atasagun
23. Sep 2019, 10:04

You’ve saved my day. Thanks so much!

24. Sep 2019, 23:49

Saved the day.

14. Oct 2019, 22:03

Thank you, you saved my evening!

28. Nov 2019, 10:44

Thank you man. You saved my day. :)

08. Jan 2020, 09:54

Thank you for this article. I've been stumped on a file upload forever. I'm still new to development, so I have a question. Where exactly do I write the delete options.headers['Content-Type']; code? Inside the fetch request?

08. Jan 2020, 09:59

Hey Dave,

fetch accepts two params, url and options. So before calling it, make sure your options are not including Content-Type header.

To answer your question, delete Content-Type header right before you call fetch.


21. Jan 2020, 09:24


Thank you for your posting. My upload is now working.

you save my evening.


29. Feb 2020, 15:15

You made my day! Thank you so much. I was having trouble in uploading photo in react native.

Faysal Bsata
02. Apr 2020, 06:21

You are an angel from god. Thank you I've wasted a day and a half trying to diagnose the issue. Thank you <3

02. Apr 2020, 09:00

You are welcome! I had no idea this is an issue for so many people, glad I could help.

26. Jun 2020, 06:10

Thank you. This post really helped. How do we upload multiple files? This is my code which does not work.

var formData = new FormData();
var fileField = document.querySelector("input[type='file']");

formData.append('file', fileField.files[0]); 
formData.append('file', fileField.files[1]); 
formData.append('file', fileField.files[2]); 

When I print the length of the fileField.files, it gives me as 1. where as I have uploaded 3 files using my html code.

How do we upload 3 files?

26. Jun 2020, 07:27

Hello Sanyukta, I think you just need to update the field name, from files to files[]. For multiple files, field needs to be an array.


30. Jun 2020, 10:51

I spent 3 hours because of that lil' contentType: 'multipart/form-data'. Luckilly I stumbled upon your post. Thank you so much

10. Sep 2020, 17:56

You must be a Frank Zappa fan! :-)

10. Sep 2020, 19:01

Indeed I’m :)

10. Sep 2020, 21:57

Wow, thanks! man, i spend some time with that boundary problem, you got a new follower.

22. Oct 2020, 00:22

I can't thank you for enough for posting this solution. Never would have thought that specifying an accurate Content-Type header field would be an issue. Saved me hours!

22. Oct 2020, 14:14

THank you so Much

24. Dec 2020, 00:09

That doesnt make sense

there is no Content-Type do delete after set options without Content-Type key

const options = { method: 'POST', body: formData }; delete options.headers['Content-Type'];

Its the same as just const options = { method: 'POST', body: formData };

25. Dec 2020, 23:26

As the above folks mentioned, your post helped me quite a bit. I spent hours till I found your post with the solution. Thank you!

26. Dec 2020, 17:42

I have spent one day for fixing this problem. I wish i could find your post sooner. Thank a lot Bro!

19. Jan 2021, 17:17

Don't working for me.

05. Feb 2021, 09:21

exactly this is what I was looking for, thanks for sharing, it solve all my issue