It checks whether the server supports the [authenticated media endpoint](https://spec.matrix.org/v1.14/client-server-api/#get_matrixclientv1mediadownloadservernamemediaid). If it does, use it. Otherwise, use the [old endpoint](https://spec.matrix.org/v1.14/client-server-api/#get_matrixmediav3downloadservernamemediaid).
Type: add