Thanks, that should work. There isn't another setting file for extensions. No need to tinker with:
/ebrmain/config/extensions.cfg
If:
/mnt/ext1/system/config/extensions.cfg
exists, it has priority.
Not sure if the device is differentiating between FLAC & flac
Just look at OwnSudoku:
https://github.com/progtologist/OwnSudoku
which adds a new filteype:
sudoku:@SUDOKU_file:1:OwnSudoku.app:ICON_TXT
into extensions.cfg and it is working with old and new firmware too.