We contacted the Windows Product Management Group and they are not aware of a place where we post/publicize file extensions. They are directing software developers to this site to register a new file extension: http://filext.com/submitext.php. Please provide the specific destination on Microsoft.com that the customer has in mind, and I would get back to the product group on it.
-brenda (ISV Buddy Team)
Our contact from the Windows Group found the customer’s best option, they recommend visiting FILExt [http://filext.com/] or UK Technical Support [http://www.uktsupport.co.uk/reference/fext/fexta.htm]. This site will be updated over time with the most commonly used file extensions. This web site only lists the most commonly used file extensions because of the costs involved with maintaining this web site in more than 20 languages.
Please have the customer direct their request to the owners of one or both of the sites above, as Microsoft does not have a path to post/publicize additional file extensions. If you have any further questions, please let me know.
I hope this helps,
-brenda (ISV Buddy Team)