code: function addMultipartData (formData, data) {
      var key, arr, i;
  
      for (key in data) {
          if (data.hasOwnProperty(key) && key !== 'files') {
              if (data[key] instanceof Array) {
                  arr = data[key];
  
                  for (i = 0; i < arr.length; i += 1) {
                      formData.append(key + '[' + i + ']', exports.stringifyIfObject(arr[i]));
                  }
              } else {
                  formData.append(key, exports.stringifyIfObject(data[key]));
              }
          }
      }
  }
docs: Helper used by `formRequestPayload` to append data to FormData instance