*.dvdr, as well as *.cdr, are essentially Apples flavour of ISO files. They are virtually identical and could even be renamed to *.iso. It would still be nice to have *.dvdr in the list of known file types in the open dialog, just to maximise compatibility.
It's also important to note that *.dvdr is not the same as the more commonly known DMG format. Apple's proprietary disk image format.