diff --git a/dlt/destinations/impl/filesystem/layout.py b/dlt/destinations/impl/filesystem/layout.py
index 9e44833d62..233a7c3643 100644
--- a/dlt/destinations/impl/filesystem/layout.py
+++ b/dlt/destinations/impl/filesystem/layout.py
@@ -186,7 +186,7 @@ def make_filename(
 
         # if extension is not defined, we append it at the end
         if "ext" not in placeholders:
-            path += f".{extras.params['ext']}"
+            path += f".{extras.file_format}"
 
         return path