different file dialog impls work differently so store the
detected filter outside the file dialog

Change-Id: I144d8df06e5265fed8f11b284a2f40c17c3e09ef
Reviewed-on: https://gerrit.libreoffice.org/45166Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>