The file magic.bacula can be added to your system files so that the 
file command will recognize Bacula Volumes as such.

If I understand this correctly, you add that to the database of
"magic" that the command file uses.
 
See 'man 1 file' and 'man 5 magic'.  On my system, there are
many ways to add to the magic.  For testing, you'd add it to
/etc/magic, usually.
   
This is an interesting contribution that might also be
relevant to the maintainers of magic, IMO.
     
Explanation by: Arno 

magic.bacula submitted in bug #715 by "shattered".