WARNING! If you change the extensions to anything different, especially system files then you risk all kinds of bad juju. Use this at your own risk.
- Backup files, place copies onto your desktop for easier editing. if you have a lot of files then make a folder on the desktop and save them in there for now
- Left click windows button, search for 'folder options,' open folder options
- in folder options go to 'view' tab, left click that tab
- scroll down a bit until you see 'hide extensions for known file types,' uncheck that option
- hit apply, leave the window open
- navigate to your backup files, change the name to song.mp3 or video.avi or whatever it was. it needs to have the exact same 3 extension container to work so if you name an avi file to mp4 it won't work
- try to see if your re-named backup opens by double clicking or using a multimedia editor to see if you can open the file for editing/saving it to a different format or editing out where the problem is.
- when you're done (whether it works or not) go back to the folder options and check 'hide extensions for known file types' back on. Hit apply and close the windows.
If it works, you're lucky. If that doesn't work then you don't have many options left.