Use a proper upload file so propfinds return 404

Fixes #20235

By using an UploadFile we make sure that we don't need to have another
check everywhere for the path. But we just have ot check (which we have
to anyway) if it is a proper Connector/File (or directory).

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>