code: function readFile() {
                      return new Promise(function (resolve, reject) {
                          /** @type {?} */
                          var reader = new FileReader();
                          reader.onload = function (event) {
                              resolve(reader.result);
                          };
                          reader.onerror = function (err) {
                              reject(err);
                          };
                          reader.readAsDataURL(file);
                      });
                  }
docs: read file from input field
  @return {?}