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
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
Returns the rendered string for the correct file type.

 @return [String]