AC-3 files not recognised

Hello all, I'm running LMS 8.1.1 on a Raspberry Pi

I have a stereo AC3 file demuxed from a video. It plays fine on my PC using VLC or foobar2000

I uploaded it to LMS using FTP along with some other FLACs and MP3s but the AC3 wasn't recognised.

Permissions and owners are unchanged and no different to the FLACs and MP3s which worked fine.

I transcoded it to FLAC on my PC and FTP'd it to the LMS - that was fine too.

I've not changed anything under Settings -> Advanced -> File Types

Any suggestions please?

Thanks for reading.