Strange,
Did you try to have 2 applications in your application group? One for XLS, and one for XLSX?
I mean, you don't need to have several application group, just an entry per type of file (Application)
What you could do, is delete the application and recreate it with the xlsx extension, without modifying the application group (that's what I do when I've messed up completely an application :-)
Sincerely yours and happy new year!
Alessandro