Better "backupexcludethumbs" regex.

Now matches @2 and 50x50.
Tests:

    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)