diff --git a/app/activitypub/util.py b/app/activitypub/util.py
index c2e5e112..210bff8e 100644
--- a/app/activitypub/util.py
+++ b/app/activitypub/util.py
@@ -981,6 +981,7 @@ def make_image_sizes_async(file_id, thumbnail_width, medium_width, directory):
                             if content_type_parts:
                                 file_ext = '.' + content_type_parts[-1]
                         else:
+                            file_ext = file_ext.replace('%3f', '?')  # sometimes urls are not decoded properly
                             if '?' in file_ext:
                                 file_ext = file_ext.split('?')[0]