Method append

src/form-data-wrapper.js:156–158

* @param {string} name * @param {string|!File} value * @param {string=} opt_filename * @override

(name, value, opt_filename)

   * @override
   */
  append(name, value, opt_filename) {
    this.formData_.append(name, value);
  }

  /** @override */
  delete(name) {