Folder Properties -> Extensions

 
If a file has an extension that is not recognised (i.e. it doesn't appear in any of the 19 categories; system, graphics, etc.) then its extension will be put in to this table. The table lists the extension and the number of files found with it.
Clicking an extension will bring up list of possible file types; e.g. in the example above, Xinoribs suggests that this is a file from Microsoft's Visual Source Safe. Xinorbis can identify over 3000 file extensions.

Use the Right Mouse Button on an extension to bring up a menu which allows you to fast-track an extension to the correct category, without the need to go in to preferences. It's possible to fast-track more than one extension at a time to the same category. Select multiple rows using SHIFT+CLICK or CONTROL+CLICK.

Clicking the "Show files" menu item will open the search tab and list all files that have the selected extension.