I haven't looked it up, but Alter,Database, and File are the three search words for BOL that comes to mind. Or if that doesn't do the trick, then a restore with a move will allow you rename those files to anything you'd like them to be. If both work, then whichever you want to do would be fine. I don't believe there's a convention on what way is "best" in this circumstance. I do agree that adding the .mdf extension to the file is "best" practice.