To be able to restore Windows file associations, you need to make a copy of two areas in the Windows registry: HKEY_CURRENT_USER\Software\Classes and HKEY_LOCAL_MACHINE\Software\Classes. Windows stores information on what application should be used to open a particular type of file, e.g. a file with a .jpg association in those two areas. If you have made a backup and some applications makes changes that you would like to reverse, you can use your backup files for those two branches of the registry to restore the file associations to what they were previously.
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