Try selecting the file in the Finder and doing File > Get Info. From there you should be able to choose any application to open it with. Once you've opened that type of file in that application once, it should start appearing in the Open With menu.
EDIT: The answer above does not work. Alternate suggestion:
I felt bad that my prev answer was incorrect, so I googled around. I found this tip from Mac OS X Hints:
- Control-click (or right-click) the app in question and select "Show Package Contents"
- Open the Contents folder in the app bundle. There you'll find a file called Info.plist. This is an XML property list that stores all sorts of information about the app.
- Open the file with your favourite text editor. I recommend Hydra, but TextEdit will do just fine.
- Search for something that looks like the following:
and so forth, with the suffices the app is able to open contained within the tags. Then you simply delete [or add --newtron] the suffix for items you don't want [or want] the app to open.
- Save the file, and close it (or quit the editor).