You can use the switch -ae. Yes, it doesn't change the extensions; it will add the guessed extension. The meaning was that this way you don't loose any info about the original file name.
The worse that can happen, is that you'll end up with a duplicated extensions, like mypicture.jpg.jpg.
Probably in a next version I will also add a switch to only change the extension if it it's wrong.
Bye!