Select the ProgID that will be registered for this file extension. If the desired ProgID is not present in the list, type your new ProgID into the ProgID field for your extension and press Enter.

A file type's ProgID is an arbitrary string, but it should be unique on the target system. One ProgID naming convention is to append the word file to your extension without a dot—the .ext extension might use the ProgID extfile. Another convention is to name a file-type ProgID after the application used to open the file type, as in SampleApp.Document.