The MythVideo application will allow you to use an external program to watch media files that are not directly supported by MythTV.
Change into the MythVideo directory, then make and make install:
$ cd mythvideo
$ qmake mythvideo.pro
$ make
$ su
# make install
# exit
The next step is to create the create the videometadata table in the database. Simply do the following:
$ cd videodb
$ mysql -u mythtv -p < metadata.sql
See MythVideo's README
file for additional information.