code: function(spec) {
              var button = this._createUploadButton({
                  accept: spec.validation.acceptFiles,
                  allowedExtensions: spec.validation.allowedExtensions,
                  element: spec.element,
                  folders: spec.folders,
                  multiple: spec.multiple,
                  title: spec.fileInputTitle
              });
  
              this._extraButtonSpecs[button.getButtonId()] = spec;
          }
docs: Creates an extra button element