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.

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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 & 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.