Well there, Dubya-Dubya...

Please try Prevx 2.0 !
Magic app.

And, here is a starter link for help in renaming file extensions...
I am not saying this is the best site or solution, but it is something that will work for those who have suffered the change of extension name from .mp3 to .zip -

Changing a file extension.


The other basic method is In Windows Explorer go to Tools/FolderOptions. Click on the View tab and if the option "Hide Extensions for Known File Types" is active, de-select it. Then proceed to rename the file, using the End key to go to the end of the filename and change the extension to "mp3."