title: react-native FormData upload does not send the right data
body: <p>I've been looking through all the questions I could find but it looks like nobody had my problem.</p>
  
  <p>I'm running react-native 39 and I want to upload an image. It looks like that it is possible to upload it using FormData.</p>
  
  <pre><code>/**
   * Polyfill for XMLHttpRequest2 FormData API, allowing multipart POST requests
   * with mixed data (string, native files) to be submitted via XMLHttpRequest.
   *
   * Example:
   *
   *   var photo = {
   *     uri: uriFromCameraRoll,
   *     type: 'image/jpeg',
   *     name: 'photo.jpg',
   *   };
   *
   *   var body = new FormData();
   *   body.append('authToken', 'secret');
   *   body.append('photo', photo);
   *   body.append('title', 'A beautiful photo!');
   *
   *   xhr.open('POST', serverURL);
   *   xhr.send(body);
   */
  </code></pre>
  
  <p>Using the method I just get [object Object] in the body of the request.
  Using fetch instead of xhr I get some data but now that I would expect, there's no image that I can get from the server</p>