Adding document/file to an existing documents via REST API

Can you show me the full HTTP request that this ends up creating?  You should be able to see it in your browser developer tools.

Here is the header, payload and the preview of the created HTTP request

Header:

Payload:

Preview:

But when I try it in POSTMAN, it successfully uploaded the file

If it works in PostMan but not from the browser then there must be some difference.  It may be worth using something like Telerik Fiddler (which is a HTTP debugging proxy) to log the two requests and see whether you can see what's different.

Okay I will it. May I also ask what is the key name of the file i want to upload? see screenshot

From recollection - I looked at this a couple of days ago when you originally posted - I think the code will iterate through any/all files in the HTTP request body, regardless of their field name.

I found the problem. I solved it by simply removing one of the headers I am passing when creating a post request. I removed the Content-type application/json header. This header is the main reason why no files is being received by the server