code: function sendFormWithFiles(form_el, on_success) {
          var on_error = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : null;
          var headers = arguments.length > 3 && arguments[3] !== undefined ? arguments[3] : { 'Content-Type': 'multipart/form-data' };
  
          this.sendForm(form_el, on_success, on_error, headers);
      }
docs: Sends a form via ajax POST with header Content-Type: multipart/form-data which is required for file uploading
  Data is automatically taken form all form input values
  
  @param {object} form_el - Form document element
  @param {callback(responseText)} on_success - callback for status code 200
  @param {callback(responseText, responseStatusCode)} on_error = null - custom handler for non 200 status codes
  @param {object} headers = {'Content-Type': 'multipart/form-data'} - key: value map of headers