Send multipart request from javascript

multipart.js

if (XMLHttpRequest.prototype.sendAsBinary === undefined) {
  XMLHttpRequest.prototype.sendAsBinary = function(string) {
    var bytes = Array.prototype.map.call(string, function(c) {
        return c.charCodeAt(0) & 0xff;
    });
    this.send(new Uint8Array(bytes));
  };
}

function sendMultiPartReqeust(filename, mimeType, imageData, message) {
  var boundary = '----ThisIsTheBoundary1234567890';
  
  var formData = '--' + boundary + '\r\n'
  formData += 'Content-Disposition: form-data; name="source"; filename="' + filename + '"\r\n';
  formData += 'Content-Type: ' + mimeType + '\r\n\r\n';
  formData += imageData;
  formData += '\r\n';
  formData += '--' + boundary + '\r\n';
  formData += 'Content-Disposition: form-data; name="message"\r\n\r\n';
  formData += message + '\r\n'
  formData += '--' + boundary + '--\r\n';

  var xhr = new XMLHttpRequest();
  xhr.open('POST', '/upload', true);
  xhr.onload = xhr.onerror = function() {
      console.log(xhr.responseText);
  };
  xhr.setRequestHeader("Content-Type", "multipart/form-data; boundary=" + boundary);
  xhr.sendAsBinary(formData);
}