Fix :attr:.ManimConfig.format not updating movie file extension (#3839)

* Fix config.format not updating config.movie_file_extension

* Add test

Author: JasonGrace2282

manim/_config/utils.py

@@ -1052,6 +1052,7 @@ def format(self, val: str) -> None:
             val,
             [None, "png", "gif", "mp4", "mov", "webm"],
         )
+        self.resolve_movie_file_extension(self.transparent)
         if self.format == "webm":
             logging.getLogger("manim").warning(
                 "Output format set as webm, this can be slower than other formats",

tests/test_config.py

@@ -4,6 +4,7 @@
 from pathlib import Path
 
 import numpy as np
+import pytest
 
 from manim import WHITE, Scene, Square, Tex, Text, tempconfig
 from manim._config.utils import ManimConfig
@@ -33,6 +34,20 @@ def test_tempconfig(config):
             assert config[k] == v
 
 
+@pytest.mark.parametrize(
+    ("format", "expected_file_extension"),
+    [
+        ("mp4", ".mp4"),
+        ("webm", ".webm"),
+        ("mov", ".mov"),
+        ("gif", ".mp4"),
+    ],
+)
+def test_resolve_file_extensions(config, format, expected_file_extension):
+    config.format = format
+    assert config.movie_file_extension == expected_file_extension
+
+
 class MyScene(Scene):
     def construct(self):
         self.add(Square())