fix for future version with AvifPlugin[ci skip]

Author: bigcat88

pillow_heif/thumbnails.py

@@ -4,7 +4,7 @@
 
 from PIL import Image
 
-from .as_opener import HeifImageFile
+from .as_opener import _LibHeifImageFile
 from .heif import HeifFile, HeifImage, HeifThumbnail
 from .private import HeifCtxAsDict
 
@@ -26,7 +26,7 @@ def thumbnail(im, min_box: int = 0):
         thumb = _choose_thumbnail(im.thumbnails, min_box)
         if thumb:
             thumb.info = im.info
-    elif isinstance(im, HeifImageFile):
+    elif isinstance(im, _LibHeifImageFile):
         thumb = _choose_thumbnail(im.info.get("thumbnails", []), min_box)
         if thumb:
             thumb = thumb.to_pillow()