Improve wording: avoid triple negative

Author: hugovk

Lib/mimetypes.py

@@ -95,9 +95,8 @@ def add_type(self, type, ext, strict=True):
         list of standard types, else to the list of non-standard
         types.
 
-        Non-empty extensions that don't start with a '.' are not
-        valid, so a ValueError will be raised if they are
-        specified.
+        Valid extensions are empty or start with a '.'.
+        Raise a ValueError for invalid extensions.
         """
         if ext and not ext.startswith('.'):
             raise ValueError("Extensions should start with a '.' or be empty")