def _get_part_reader(
        self,
        headers: "CIMultiDictProxy[str]",
    ) -> Union["MultipartReader", BodyPartReader]:
        """Dispatches the response by the `Content-Type` header.

        Returns a suitable reader instance.

        :param dict headers: Response headers
        """
        ctype = headers.get(CONTENT_TYPE, "")
        mimetype = parse_mimetype(ctype)

        if mimetype.type == "multipart":
            if self.multipart_reader_cls is None:
                return type(self)(headers, self._content)
            return self.multipart_reader_cls(headers, self._content)
        else:
            return self.part_reader_cls(self._boundary, headers, self._content)
Dispatches the response by the `Content-Type` header.

        Returns a suitable reader instance.

        :param dict headers: Response headers
        
lib/aiohttp/multipart.py