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
$ make
$ su
# make install
# exit
The next step is to edit the
/usr/local/share/mythtv/mythvideo-settings.txt
file. Change
the StartDir to a valid mount point on your system.