In the suggested format for the generic Open
!MP3!MID!KAR!FLAC!WAV!CDA!OGG!WMA!.BAT
the choice of the separator, '!' in this case, is up
for discussion, of course.
Also, the leading and trailing '!' seem superflous,
what's wrong with removing them:
MP3!MID!KAR!FLAC!WAV!CDA!OGG!WMA.BAT
My reasoning is that the latter format needs more parsing
code, because you have two separators to think about,
'!' and '.', but it's probably a trivial coding problem.
With the first format, you just look for a first matching
'!ext!' substring in the file spec base part -- but coding
is none of my business, of course.
However, the main reason for this choice is that, if a file
starts with '!', it will sort with other similar generic Open
batch files. If this convention is adopted, such files will
be easier to spot as a category, provided the user does not
use '!' in their private files a lot.
Also, '!' is the very first printable ASCII character, so
these files will be at top in File View (if sorted by Name).