Assert image type

Tests/test_file_gif.py

@@ -1163,6 +1163,7 @@ def test_append_images(tmp_path: Path) -> None:
     im.copy().save(out, append_images=ims)
 
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
         assert reread.n_frames == 3
 
     # Tests appending using a generator

Tests/test_file_wmf.py

@@ -103,10 +103,12 @@ def test_load_set_dpi() -> None:
 
         if not hasattr(Image.core, "drawwmf"):
             return
+        assert isinstance(im, WmfImagePlugin.WmfStubImageFile)
         im.load(im.info["dpi"])
         assert im.size == (1625, 1625)
 
     with Image.open("Tests/images/drawing.emf") as im:
+        assert isinstance(im, WmfImagePlugin.WmfStubImageFile)
         im.load((72, 144))
         assert im.size == (82, 164)