Associates a file name extension with a ProgID.
HKEY_LOCAL_MACHINE\SOFTWARE\Classes .ext = ProgID
The information contained in the file name extension key is used by both the Windows Explorer and file monikers. GetClassFile uses the file name extension key to supply the associated CLSID.
The HKEY_LOCAL_MACHINE\SOFTWARE\Classes key corresponds to the HKEY_CLASSES_ROOT key, which was retained for compatibility with earlier versions of COM.