code: def register_upload_callback(self, *args, **kwargs):
          
          if 'args_extra' not in kwargs:
              kwargs['args_extra'] = [request.files]
          return sijax.plugin.upload.register_upload_callback(self._sijax, *args, **kwargs)
docs: Registers an Upload function (see :ref:`upload-plugin`)
          to handle a certain form.
  
          Refer to :func:`sijax.plugin.upload.register_upload_callback`
          for more details.
  
          This method passes some additional arguments to your handler
          functions - the ``flask.request.files`` object.
  
          Your upload handler function's signature should look like this::
  
              def func(obj_response, files, form_values)
  
          :return: string - javascript code that initializes the form