FileType --
http://www.frederikseiffert.de/filetype/
Free, and it works on OS 9 too.
The culprit I recall that changed the mappings was one of the versions of RealPlayer for Mac.

The last time I used that (years ago) was the last time I used that
http://www.micromat.com/tt_lite/tt_lite.html was also handy for rebuilding the desktop database file and fixing lost mappings. I see it's still free.