Sorts by format before filename

I have a large gallery containing a mix of photos in portrait and landscape formats. The BG sort option displays all the portrait photos first and then all the landscape ones. Within the set of portrait photos, the photos are sorted by filename. Within the set of landscape photos, the photos are sorted by filename. But what I want (and I imagine what most people want) is for the whole set to be sorted by filename (with the portrait and lansdscape photos mixed up).