diff --git a/csst_common/file.py b/csst_common/file.py
index 6727cfcdf06d81d57e1d01c01524c3fd519f66d9..009e1f451da8920cf014725b634da61262702e91 100644
--- a/csst_common/file.py
+++ b/csst_common/file.py
@@ -11,18 +11,14 @@ class File:
         self.new_dir = new_dir if new_dir is not None else self.dirname
 
     def replace_ext(
-        self, new_ext: str = "wht.fits", new_dir: Optional[str] = None
+        self, new_ext: Optional[str] = "img.fits", new_dir: Optional[str] = None
     ) -> str:
         if new_dir is None:
             new_dir = self.new_dir
+        if new_ext is None:
+            new_ext = self.ext
 
         if new_ext.startswith("."):
-            return os.path.join(
-                new_dir if new_dir is not None else self.dirname,
-                self.prefix + new_ext,
-            )
+            return os.path.join(new_dir, self.prefix + new_ext)
         else:
-            return os.path.join(
-                new_dir if new_dir is not None else self.dirname,
-                self.prefix + "_" + new_ext,
-            )
+            return os.path.join(new_dir, self.prefix + "_" + new_ext)