This is copied from the official apple docs: https://developer.apple.com/documentation/avfoundation/avassettrack/formatdescriptions#Discussion

I don't know what format is in the original ffmpeg implementation. It's not documented anywhere and when I've tested it the ffmpeg implementation didn't even return it. Because it's later compared to "mp4" to check if format is correct I'm just returning the file extension. We can run more complicated checks but I think it's good enough for our usecase.

According to the docs link you posted above, isn't this way of accessing format descriptions deprecated?

It is deprecated but the new way is available for ios 15+ and we're supporting ios 13+