How to get or set boundary in multipart/form-data from FormData?

As in topic: I would like to set up my own boundary parameter for Content-Type when I work with FormData class in JavaScript.

Expected header:

xxxxxxxxxx

Content-Type: multipart/form-data; boundary=------some-random-characters

My source code looks in the folowing way:

xxxxxxxxxx

const requestData = new FormData();

requestData.append('file', file); // file from File API

​

const response = await fetch('/backend/upload', {

    method: 'POST',

    headers: {

        'Content-Type': 'multipart/form-data; boundary=------some-random-characters',

        'Accept': 'application/json'

    },

    body: requestData

});

var responseData = await response.json();

When I execute the source code, header Content-Type is correct, but in body I have:

------WebKitFormBoundary2lZSUsxEA3X5jpYD instead of ------some-random-characters.

Request screenshot from Google Chrome DevTools:

Any idea how to get or set boundary for FormData?