Return list of supported extensions when an unknown one is provided

scripts/greaseweazle/tools/util.py

@@ -171,7 +171,7 @@ def get_image_class(name):
     else:
         _, ext = os.path.splitext(name)
         error.check(ext.lower() in image_types,
-                    "%s: Unrecognised file suffix '%s'" % (name, ext))
+                    "%s: Unrecognised file suffix '%s' (known suffixes: %s)" % (name, ext, ', '.join(map(lambda t: t[0], image_types))))
         typename = image_types[ext.lower()]
     mod = importlib.import_module('greaseweazle.image.' + typename.lower())
     return mod.__dict__[typename]