def mimetype_params(self) -> t.Mapping[str, str]:
        """The mimetype parameters as dict.  For example if the
        content type is ``text/html; charset=utf-8`` the params would be
        ``{'charset': 'utf-8'}``.

        .. versionadded:: 0.14
        """

        def on_update(d: CallbackDict[str, str]) -> None:
            self.headers["Content-Type"] = dump_options_header(self.mimetype, d)

        d = parse_options_header(self.headers.get("content-type", ""))[1]
        return CallbackDict(d, on_update)
The mimetype parameters as dict.  For example if the
        content type is ``text/html; charset=utf-8`` the params would be
        ``{'charset': 'utf-8'}``.

        .. versionadded:: 0.14
        
src/werkzeug/test.py