File names are presented after the folders, and are sorted alphabetically independent of case.
The buttons on this panel are fairly self-explanatory.
File type selector The button that may be non-obvious is the "ComboBox" in the middle at the bottom (next to "Files of type:"): this toggles between selecting for the file suffixes specified for the "diagram language" and "All" (all suffixes) - just use the mouse to select the desired option, or use the keyboard (tab buttons and up and down arrows).