.DMG File Association

I've seem to lost the ability to automount a .dmg file. Hexedit seems to think that it owns these files. So, I control-click on the archive in question, select DiskImageMounter, and everything is golden. The problem is that I cannot find that program on my system (that is, it is not a selectable program when I choose to make the relationship permanent). Is DiskImageMounter just a sub-program of Disk Utility or what?