code: def append_and_render
        loaded_list = SubscriptionListParser.new(list: destination)
        case File.extname destination
        when '.xml' || '.opml'
          render_opml(loaded_list)
        when '.json'
          render_json(loaded_list)
        when '.txt'
          render_txt(loaded_list)
        end
      end
docs: Returns the rendered string for the correct file type.
  
   @return [String]