Add quotes around bad format to help when there is none.

Author: jonathanrocher

stage4.1_first_application/pycasa/ui/tasks/pycasa_task.py

@@ -50,4 +50,4 @@ def open_in_central_pane(self, filepath):
             obj = ImageFile(filepath=filepath)
             self.central_pane.edit(obj, factory=ImageFileEditor)
         else:
-            print("Unsupported file format: {}".format(file_ext))
+            print("Unsupported file format: '{}'".format(file_ext))