diff --git a/csst_common/file.py b/csst_common/file.py
index c81b966999d4ac451f4e6b130d13698048ed2c68..6727cfcdf06d81d57e1d01c01524c3fd519f66d9 100644
--- a/csst_common/file.py
+++ b/csst_common/file.py
@@ -1,13 +1,28 @@
 import os
+from typing import Optional
 
 
 class File:
-    def __init__(self, file_path: str = "/path/to/file.fits"):
+    def __init__(self, file_path: str = "/path/to/file.fits", new_dir=None):
         self.file_path = file_path
-        self.file_path_prefix, self.file_path_ext = os.path.splitext(file_path)
+        self.dirname = os.path.dirname(self.file_path)
+        self.file_name = os.path.basename(self.file_path)
+        self.prefix, self.ext = os.path.splitext(self.file_name)
+        self.new_dir = new_dir if new_dir is not None else self.dirname
 
-    def replace_ext(self, ext="wht.fits") -> str:
-        if ext.startswith("."):
-            return self.file_path_prefix + ext
+    def replace_ext(
+        self, new_ext: str = "wht.fits", new_dir: Optional[str] = None
+    ) -> str:
+        if new_dir is None:
+            new_dir = self.new_dir
+
+        if new_ext.startswith("."):
+            return os.path.join(
+                new_dir if new_dir is not None else self.dirname,
+                self.prefix + new_ext,
+            )
         else:
-            return self.file_path_prefix + "_" + ext
+            return os.path.join(
+                new_dir if new_dir is not None else self.dirname,
+                self.prefix + "_" + new_ext,
+            )