diff --git a/csst_common/file.py b/csst_common/file.py
index c1a04776b33384f5207a69bcb36fd3787f7625c3..2d4ae3660b972e6496e838622ca38d36c1c708dc 100644
--- a/csst_common/file.py
+++ b/csst_common/file.py
@@ -55,26 +55,44 @@ class File:
             r"(?P<ext>[a-z.]+)"
         )
         self.mo = re.fullmatch(pattern, self.file_name)
-        assert self.mo is not None, f"Pattern does not match for file: {self.file_name}"
+        assert self.mo is not None, f"Pattern does not match: {self.file_name}"
         # set attributes
-        for k, v in self.mo.groupdict().items():
-            self.__setattr__(k, v)
+        # for k, v in self.mo.groupdict().items():
+        #     self.__setattr__(k, v)
+        self.telescope = self.mo.groupdict()["telescope"]
+        self.instrument = self.mo.groupdict()["instrument"]
+        self.project = self.mo.groupdict()["project"]
+        self.obs_type = self.mo.groupdict()["obs_type"]
+        self.exp_start = self.mo.groupdict()["exp_start"]
+        self.exp_stop = self.mo.groupdict()["exp_stop"]
+        self.obs_id = self.mo.groupdict()["obs_id"]
+        self.detector = self.mo.groupdict()["detector"]
+        self.level = self.mo.groupdict()["level"]
+        self.version = self.mo.groupdict()["version"]
+        self.ext = self.mo.groupdict()["ext"]
 
     def derive(
         self,
-        new_dir=None,
-        telescope=None,
-        instrument=None,
-        project=None,
-        obs_type=None,
-        exp_start=None,
-        exp_stop=None,
-        obs_id=None,
-        detector=None,
-        level=None,
-        version=None,
-        ext=None,
+        ext: Optional[str] = None,
+        new_dir: Optional[str] = None,
+        telescope: Optional[str] = None,
+        instrument: Optional[str] = None,
+        project: Optional[str] = None,
+        obs_type: Optional[str] = None,
+        exp_start: Optional[str | int] = None,
+        exp_stop: Optional[str | int] = None,
+        obs_id: Optional[str | int] = None,
+        detector: Optional[str | int] = None,
+        level: Optional[str | int] = None,
+        version: Optional[str | int] = None,
     ):
+        if ext is None:
+            ext = self.ext
+        else:
+            assert isinstance(ext, str)
+            if not ext.startswith("."):
+                ext = "_" + str(ext)
+
         return os.path.join(
             new_dir if new_dir is not None else self.new_dir,
             f"{self.telescope if telescope is None else str(telescope)}_"
@@ -87,7 +105,7 @@ class File:
             f"{self.detector if detector is None else str(detector)}_"
             f"L{self.level if level is None else str(level)}_"
             f"V{self.version if version is None else str(version).zfill(2)}"
-            f"{self.ext if ext is None else str(ext)}",
+            f"{ext}",
         )
 
     def derive0(self, *args, **kwargs):