To do so, just type an asterisk (which Windows takes as a wildcard) followed by the file extension you're looking for in the File name field and hit Enter. For example, typing *.jpg will isolate all of your JPEGs. To undo the filter, just type a lone asterisk and hit Enter. Actually, you can take this even further.
If you really want to get fancy, try typing an asterisk and then part of a word you know is in the filename and then another asterisk (e.g., *birthday*), which will show you all files in that directory with "birthday" somewhere in the filename. In most cases, though, if you know the file that well you may as well just start typing the beginning of the filename and pick from the matches in the drop-down. Thanks David!
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