ui-snapshot.c: change mime-type for tar.gz and tar.bz2

The updated mime-types seems to work better than the old ones with both
safari and firefox.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

tests/t0107-snapshot.sh

@@ -10,7 +10,7 @@ run_test 'get foo/snapshot/test.tar.gz' '
 
 run_test 'check html headers' '
 	head -n 1 trash/tmp |
-	     grep -e "Content-Type: application/x-tar" &&
+	     grep -e "Content-Type: application/x-gzip" &&
 
 	head -n 2 trash/tmp |
 	     grep -e "Content-Disposition: inline; filename=.test.tar.gz."