After trying with a bunch of different files, it's interesting that some .img were given 'Disk Copy document' which opens with 6.3.3 while others were given 'Disk Copy 4.2 disk image' which still opens with 6.3.3. Along a similar vein, .sit were given a variety from 'Stuffit 1.5.1 archive', 'Stuffit Deluxe 3 archive', or 'Stuffit Expander document'.
Fix-a-Fork didn't seem to do anything to .dmg .cdr .iso .image .toast .jpg .html or .png files. I would guess .dmg and .image goes to Disk Copy, where .cdr .iso .toast goes to Virtual CD/DVD Utility, and .html .png .jpg goes to browser?