code: function(id) {
              var buttonId;
  
              if (qq.supportedFeatures.ajaxUploading) {
                  buttonId = this._handler.getFile(id).qqButtonId;
              }
              else {
                  buttonId = this._getButtonId(this._handler.getInput(id));
              }
  
              if (buttonId) {
                  this._buttonIdsForFileIds[id] = buttonId;
              }
          }
docs: Maps a file with the button that was used to select it.