def METHOD_NAME(self):
    """Files with extensions listed in app_static_file_handler.py
    `SAFE_APP_STATIC_FILE_EXTENSIONS` (e.g. png) should have the
    `Content-Type` header based on their extension.
    """
    response = self.fetch(f"/app/static/{self._png_image_filename}")
    assert response.code == 200
    assert response.headers["Content-Type"] == "image/png"
    assert response.headers["X-Content-Type-Options"] == "nosniff"