Python must have libav bindings somewhere, you could certainly run that check in-process. Off the top of my head, it's probably in the container metadata, so y

Python must have libav bindings somewhere, you could certainly run that check in-process. Off the top of my head, it's probably in the container metadata, so you'd just need libavformat and not even libavcodec. Pass it a path, open it, scan the list of streams and check the codec magic number?