If you drag a file from the OSX Finder to the various icon on the Dock, you'll notice that only certain apps will highlight to show that they will accept that type of file. However, sometimes you want to force an application to open a type of file, even if it doesn't "know" about it. To do this. hold down command and option when you drag the file. You should be able to drop the file on any application.
Mac OS X Hints