Better "backupexcludethumbs" regex.

Matches these:
0.0_Frontpage-1024x833.jpg
0.0_Frontpage-1024x833@2x.jpg (Not matched before)
0.0_Frontpage-737x600.jpg
0.0_Frontpage-737x600@2.jpg  (Not matched before)
0.0_Frontpage-100x100.jpg
0.0_Frontpage-50x50.jpg  (Not matched before)
0.0_Frontpage-50x50@2.jpg  (Not matched before)