RegisterFileType: optional argument 'ProgId' not documented

The RegisterFileType command has an undocumented option 'ProgId' to give a unique and identifiable name to the file association.

I came across this by chance when searching for example usages of the command. It can be very handy if there are multiple applications in a package that all use the same extension (xxx). Using unique ProgId values for each registration, allows multiple "OpenWithProgId". Without it, the second registration effectively overwrites the first (because both are registered with ProgId 'xxx_auto_file'.