code: def use(self, obj, parent_form=None):
          
  
          if not isinstance(obj, self.input_classes):
              raise RuntimeError('{0!s} cannot handle a {1!s}'.format(self.__class__.__name__, obj.__class__.__name__))
  
          self.parent_form = parent_form
  
          if self.title is None:
              self.title = 'file: '+obj.filename
          self._do_use(obj)
docs: Note: if title is None, will be replaced with obj.filename