[Google-Desktop-Developer] How to modify the file list dropdown on the serach results page

When the search results page is displayed, a number of hyperlinks are
available: All, Emails, Files, ... If I click on the Files link I get a
subset of the results with a dropdown control at the top of the search
to further filter by file type.

How can I control the content of that dropdown control to add new file
types?

Thanks