code: def set_thumbnail(self, thumbnail):
          
          self.thumbnail = thumbnail
          if isinstance(self.thumbnail, str):
              from .files import ThumbnailFile
              self.thumbnail = ThumbnailFile(path=self.thumbnail)
  
          if self.thumbnail:
              self.add_file(self.thumbnail)
docs: set_thumbnail: Set node's thumbnail
              Args: thumbnail (ThumbnailFile): file model to add to node
              Returns: None