How to filter by extensions in the advanced search

Hi SAP gurus

I would like to enable end users to filter their search results by extensions so that the search result only would

include .doc, .xls, .mpp, .htm/html, .pdf and/or .ppt-documents.

How do you enable this functionality in the Advanced Search window?

If I select cm_extension in the SearchOptionsSet, I dont get any option in the "Predefined Property"-field in the Advanced Search window.

Any help is greatly appreciated and will be rewarded.

Best regards,

Martin