GTKtalog uses MIME types to store the file types. So does Gnome, Kde ... The MIME type is determined by the extension or (best) by the content.
A MIME type is a string such as "application/zip" for (.zip files) or "image/jpeg" for (.jpg files)
In four places of GTKtalog, you are asked to type a Mime type (all in the preferences):
Adding/Editing a plugin
Adding/Editing a vfs
Mime types to skip during scan
Mime types icons
If there is no known MIME type for your extension, you'll have to add it in the Gnome control center : Document Handlers/File types and programs. See the Gnome Control Center documentation for more information.