So I'm following this tutorial
https://library.morph.zone/Creating_Ambient_Filetypes

Under System:Prefs/Ambient/filetypes I made a new subfolder audio and in this folder
I made a file called ym

AMTD
1
Name YM Chip Register Dump
Type audio/x-ym
Match
PatternHint #?.ym
End
End

Rebooted for good measure but the filetype does not seem to be recognised ?
I see it listed under audio in the Ambient Mime settigns but when clicking on a file to configure the Filetype actions , filetype is not recognised ?

Should the file be named x-ym or the Type in the file or am I doing something else wrong ?