I renamed the file to .mpg and I saw MediaMonkey tried to read the metadata, but MediaMonkey got stuck reading the file.
So I renamed it back and renamed an existing file with the .mpg extension to .m1v and the file was no longer in my library (neither as .mpg nor as .m1v).

Hi,
Have you tried to Rename that file to .MPEG?

I've got a m1v file on my computer, but MediaMonkey doesn't add it to my videos when scanning the folder. Neither does it get added as an audio file. It doesn't get scanned at all.
It's a valid mpeg 1 video (no audio)