There is a file type modifier and there are categories too... They're, unfortunately, not known to the common user because there is no help dialog or information in the popup other than datemodified and size which show in the dialog.. If you've used the modifier in the past, it should appear when you start typing, provided the first letter aligns with the first letter of the modifier..
To look for a kind of file, use kind: or kinds: followed by your selection.
Note: The following list of selections will show ( Calendar, Communication, Contact, Document, E-Mail, Feed, Folder, Game, Instant Message, Journal, Link, Movie, Music, Note, Picture, Program, Recorded TV, Saved Search, Task, Video, and Web History )
For types ( extensions ) use type: followed by the extension.
You can also add tags to files; these are keywords YOU define. To look for these, use tag: or tags: followed by your keywords.
To search for a file author, use author: or authors: followed by the author.
There are many more search features at your disposal such as logical keywords ( AND, NOT, etc.. ), other identifiers / tags such as those listed above to help narrow your search. Please refer to the following site for a, hopefully, full workup of the search features:
Tips for finding files