Getting the bindings back is as simple as Option-i on an image and change the Open with setting. But what if you cannot see the DiskImageMounter application and is not in the list anymore?
I just ran a quick search, and the path is:
If you are bored, you should take a look in this folder. It seems to contain a bunch of useful things
UPDATED: If the Finder does not allow the Change All option, you might try to fix the disk permissions using the Disk Utility.