Apply Steve advice

Author: rruuaanng

Lib/mimetypes.py

@@ -90,8 +90,8 @@ def add_type(self, type, ext, strict=True):
         list of standard types, else to the list of non-standard
         types.
         """
-        if not type and ext in self.types_map[strict]:
-            type = self.types_map[strict][ext]
+        if not type:
+            return
         self.types_map[strict][ext] = type
         exts = self.types_map_inv[strict].setdefault(type, [])
         if ext not in exts: