The Select Files dialog has two modes; simple and advanced. It is accessed with the Edit / Select by Pattern command. The dialog remembers which mode it was last used in and will open in that mode the next time it is used; you can switch modes using the Advanced or Simple button at the bottom of the dialog.
The simple-mode Select Files dialog lets you select files in the current file display by typing in a wildcard string using the standard pattern matching syntax. In the example shown above, the string *.(mp3|m4a) will select all files that end in either .mp3 or .m4a, when you click the OK button.
The Partial match option enables partial matching of the string you enter - for example, kit would match Cute Kitten.jpg if that option was turned on.
You can click the Advanced button to go to advanced mode, which lets you use more complex filters to select files.