KFileWidget::setMimeFilter() documentation says a default mime filter
should be set if the dialog is being open in save mode.
Without a default filter, the dialog uses a custom item (created by
concatenating all the mime type filters) as default. This is not useful
because it means an additional click for the user to select a filter that can
actually be useful.
If the initiallySelectedMimeTypeFilter() is empty, we use the first mime
filter as fallback.