code: def get_download_url(self, instance, default=None):
          
          download = default
          # calculate the download url
          download = "{url}/@@download/{fieldname}/{filename}".format(
              url=api.get_url(instance),
              fieldname=self.get_field_name(),
              filename=self.get_filename(instance),
          )
          return download
docs: Calculate the download url