docs: def get_response_content_type(self):
          """Figure out what content type will be used in the response."""
          if self._best_response_match is None:
              settings = get_settings(self.application, force_instance=True)
              acceptable = headers.parse_accept(
                  self.request.headers.get(
                      'Accept',
                      settings.default_content_type
                      if settings.default_content_type else '*/*'))
              try:
                  selected, _ = algorithms.select_content_type(
                      acceptable, settings.available_content_types)
                  self._best_response_match = '/'.join(
                      [selected.content_type, selected.content_subtype])
                  if selected.content_suffix is not None:
                      self._best_response_match = '+'.join(
                          [self._best_response_match, selected.content_suffix])
              except errors.NoMatch:
                  self._best_response_match = settings.default_content_type
  
          return self._best_response_match