win32 fpicker: correctly set the default file name

Correctly set the default file name in win2k+
See fdo#74295 for more details.

Change-Id: I30117a8e1e50b157fee96f4777b0c3f11630165c
Reviewed-on: https://gerrit.libreoffice.org/10430
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>