Assert image type

Tests/test_file_fli.py

@@ -176,6 +176,7 @@ def test_seek_tell() -> None:
 
 def test_seek() -> None:
     with Image.open(animated_test_file) as im:
+        assert isinstance(im, FliImagePlugin.FliImageFile)
         im.seek(50)
 
         assert_image_equal_tofile(im, "Tests/images/a_fli.png")

Tests/test_file_gif.py

@@ -407,6 +407,7 @@ def test_save_netpbm_l_mode(tmp_path: Path) -> None:
 
 def test_seek() -> None:
     with Image.open("Tests/images/dispose_none.gif") as img:
+        assert isinstance(img, GifImagePlugin.GifImageFile)
         frame_count = 0
         try:
             while True: