Post file/blob using multipart formdata ?

In the browser, if I do something like:

    var headers = {Authorization: "Basic " + btoa("user:pass")};
    // Build form data
    var formData = new FormData();
    // File obtained via File API
    formData.append('attachment', file, file.name);
    // Some data as JSON
    formData.append('data', JSON.stringify({foo: "bar"}));
    // Post using GlobalFetch API
    fetch(url, {method: "POST", body: formData, headers: headers})
      .then(...)

The request issued in the browser will have Content-Type:"multipart/form-data; boundary=---------------------------<random-number> and the body will be like:

-----------------------------<random-number>
Content-Disposition: form-data; name="attachment"; filename="velo.jpg"
Content-Type: image/jpeg

ÿØÿà ....

-----------------------------<random-number>
Content-Disposition: form-data; name="data"

{"foo":"bar"}
-----------------------------<random-number>--

I would also work (from the browser) using blobs:

const blob = new Blob([new Uint8Array(array)], {type: "image/jpeg"});
formData.append('attachment', blob, "velo.jpg");

For our integration tests we’re using isomorphic-fetch@2.2.1 and jsdom@9.4.1 and we can’t get obtain the same behavior (content-type and body seem to remain empty). I could not find any existing issue that was specific to multiparts (#30 is about FormData).

Is that something reasonably feasible ? Can we help ?

Thanks a lot!