Hi, Jacoba, The app extension is an actual application file, just like Safari, iChat, etc. The .dmg files are images, and are equivalent to things like a virtual CD in your drive. You should not be seeing the .app extension there. Normally OS X hides that, and that definitely may be a great place to start. Thanks for sharing that information! In the meantime, please see the response to your e-mail support request, and if you don't mind, I may pick your brain on this a little more as I continue to investigate and solve this issue so we can prevent it in the future.