docs: def _add_content_type(self, partname, content_type):
          """
          Add a content type for the part with *partname* and *content_type*,
          using a default or override as appropriate.
          """
          ext = partname.ext
          if (ext.lower(), content_type) in default_content_types:
              self._defaults[ext] = content_type
          else:
              self._overrides[partname] = content_type