Do not load image during save if file extension is unknown

Tests/test_file_hdf5stub.py

@@ -43,7 +43,7 @@ def test_save() -> None:
     # Arrange
     with Image.open(TEST_FILE) as im:
         dummy_fp = BytesIO()
-        dummy_filename = "dummy.filename"
+        dummy_filename = "dummy.h5"
 
         # Act / Assert: stub cannot save without an implemented handler
         with pytest.raises(OSError):

src/PIL/Image.py

@@ -2510,13 +2510,6 @@ class Image:
             # only set the name for metadata purposes
             filename = os.fspath(fp.name)
 
-        # may mutate self!
-        self._ensure_mutable()
-
-        save_all = params.pop("save_all", False)
-        self.encoderinfo = {**getattr(self, "encoderinfo", {}), **params}
-        self.encoderconfig: tuple[Any, ...] = ()
-
         preinit()
 
         filename_ext = os.path.splitext(filename)[1].lower()
@@ -2531,6 +2524,13 @@ class Image:
                 msg = f"unknown file extension: {ext}"
                 raise ValueError(msg) from e
 
+        # may mutate self!
+        self._ensure_mutable()
+
+        save_all = params.pop("save_all", False)
+        self.encoderinfo = {**getattr(self, "encoderinfo", {}), **params}
+        self.encoderconfig: tuple[Any, ...] = ()
+
         if format.upper() not in SAVE:
             init()
         if save_all: