Function handle_attachment_file 

pub(super) async fn handle_attachment_file(
    __arg0: State<AppState>,
    __arg1: Path<(String, String)>,
    headers: HeaderMap,
) -> Response

GET /api/v1/attachments/:id/:kind. The original is immutable by hash; thumb and send-copy change with the transform fingerprint, so everything is no-cache with an ETag of <id>-<fp> and a 304 on a match.