Make it so it accepts blah.tiff as a filename not just blah.tif

This is for bug [#15132] When saving a render .tif is appended to the saved 
	file even if the name contains .tiff

Kent

source/blender/blenkernel/intern/image.c

@@ -857,8 +857,8 @@ void BKE_add_image_extension(char *string, int imtype)
 			extension= ".bmp";
 	}
 	else if(G.have_libtiff && (imtype==R_TIFF)) {
-		if(!BLI_testextensie(string, ".tif"))
-			extension= ".tif";
+		if(!BLI_testextensie(string, ".tif") && 
+			!BLI_testextensie(string, ".tiff")) extension= ".tif";
 	}
 #ifdef WITH_OPENEXR
 	else if( ELEM(imtype, R_OPENEXR, R_MULTILAYER)) {