How do you specify the type of file you're looking for in an 'open file' dialog with a pattern?
You have Spotlight available from every file-picker dialog.



You can search by name or extension/file type, either in the currently open directory or the entire machine.



Sidebar hidden to save unnecessary extraneous detail

If you know the name rather than the filetype of the document you are looking for, then, same as in any Finder folder, you can start to type the name &amp; the selection will jump to the first match for that string.

eg typing t,e,s will find the first match on test.txt, but you can't use this to find a file type .xml etc as it only searches from the beginning of the name.