If Backup is enabled, Advanced HTML Optimizer will copy the whole tree structure with the original HTML files to the specified Backup folder. In this case the backup files will still keep their original file extensions. They are only transferred to another location on your system. So it's easy to restore them with any windows file manager.
Note: You can disable creating backups and specify a new Backup folder in the Options Window.
Be aware: Like in any other application, a backup file is always created from the last state of your HTML file! If you are just playing around with Advanced HTML Optimizer (we know many users love to do this), don't forget to restore your files every time.