diff --git a/atef/widgets/config.py b/atef/widgets/config.py
index 010b492e..0ef457c5 100644
--- a/atef/widgets/config.py
+++ b/atef/widgets/config.py
@@ -185,6 +185,8 @@ def save_as(self, *args, filename: Optional[str] = None, **kwargs):
                 caption='Save as',
                 filter='Json Files (*.json)',
             )
+        if not filename.endswith('.json'):
+            filename += '.json'
         try:
             with open(filename, 'w') as fd:
                 json.dump(serialized, fd, indent=2)