If you don't want to deal with extensions at all, configure Windows to hide them. Double-click My Computer (or go to the main page showing all your files for an external hard drive), click Tools, and then click Folder Options. Select the View tab and scroll down the list until you see an option that says Hide MS-DOS File Extensions For Known File Types (or simply Hide Extensions For Known File Types) and put a check in the box next to that entry. Click Apply and the extensions disappear from all of your files. Now you can rename anything without typing an extension, and Windows will automatically retain the extension that existed before you renamed the file.