Added the .tif extension as supported filetype.
Added the .tif extension as supported filetype.

[Fixes bug #676099]

Mailaender/Pinta,jakeclawson/Pinta,Fenex/Pinta,Fenex/Pinta,jakeclawson/Pinta,PintaProject/Pinta,PintaProject/Pinta,Mailaender/Pinta,PintaProject/Pinta