I changed the icon image of the ".bak" files and I don't know how to restore it to the default one. I tried to delete the file "desktop.ini", without success.
that depends on your windows installation and whether you have other programs generating .bak files (which might use different icons) Once you have determined which program to use to open .bak files, windows will associate the icon of that program with the .bak file extension.
In this blog there are a few steps you can follow to 'un-associate' a .bak file from a default software, the icon will then look like a blank page.
