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);