fdo#74064 The filename gets properly highlighted in the Save As dialog.

The filename was missing the extension which was causing problems.
The extension is retained now by disabling the autoextension checkbox.

Change-Id: If913124c22265da2bacb043d83386fd8f5710ca8
Reviewed-on: https://gerrit.libreoffice.org/8364
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>