def get_send_file_max_age(self, filename: str | None) -> int | None:
        """Used by :func:`send_file` to determine the ``max_age`` cache
        value for a given file path if it wasn't passed.

        By default, this returns :data:`SEND_FILE_MAX_AGE_DEFAULT` from
        the configuration of :data:`~flask.current_app`. This defaults
        to ``None``, which tells the browser to use conditional requests
        instead of a timed cache, which is usually preferable.

        Note this is a duplicate of the same method in the Flask
        class.

        .. versionchanged:: 2.0
            The default configuration is ``None`` instead of 12 hours.

        .. versionadded:: 0.9
        """
        value = current_app.config["SEND_FILE_MAX_AGE_DEFAULT"]

        if value is None:
            return None

        if isinstance(value, timedelta):
            return int(value.total_seconds())

        return value  # type: ignore[no-any-return]
Used by :func:`send_file` to determine the ``max_age`` cache
        value for a given file path if it wasn't passed.

        By default, this returns :data:`SEND_FILE_MAX_AGE_DEFAULT` from
        the configuration of :data:`~flask.current_app`. This defaults
        to ``None``, which tells the browser to use conditional requests
        instead of a timed cache, which is usually preferable.

        Note this is a duplicate of the same method in the Flask
        class.

        .. versionchanged:: 2.0
            The default configuration is ``None`` instead of 12 hours.

        .. versionadded:: 0.9
        
src/flask/app.py