Entries in this subkey map the Macintosh file type and creator tags to MS-DOS eight-plus-three filenames. This is the Registry path for this subkey:
HKEY_CURRENT_USER\Software\Microsoft\Mail\Mac FileTypes
Mail uses these entry values to determine what application to launch on a file attachment that has been sent from a Macintosh mail client. There are two alternate forms for the entries:
creator:type=extension or :type=extension
Both the creator and type are sequences of four characters (possibly including blanks). For example,
:TEXT=DOC
launches the application associated with the extension .DOC (Word for Windows, for example) on any Macintosh file of type TEXT.