In the Windows version (tested in v3.71), the FOC has two drop-down list boxes:

Open file name
List files of type

In the usual implementation of drop-down list boxes, when the list box has focus one can expand the list box by using the key shortcut <Alt+CurDn>, but this key shortcut does nothing on these two drop-down list boxes in the Windows version (it does work OK in the OS/2 version).

The drop-down list's control arrow on the right works fine.