files/models.py

@@ -203,8 +203,9 @@ class File(CreatedModifiedMixin, TrackChangesMixin, models.Model):
         Fall back to the source file, if no thumbnail is stored.
         Log absence of the thumbnail file instead of exploding somewhere in the templates.
         """
-        if not self.is_image and not self.is_video:
-            return ''
+        # We don't (yet?) have thumbnails for anything other than images and videos.
+        assert self.is_image or self.is_video, f'File pk={self.pk} is neither image nor video'
+
         try:
             path = self.metadata['thumbnails'][size_key]['path']
             return self.thumbnail.storage.url(path)