dont try to extract frames from audio files

pandora/item/models.py

@@ -1097,6 +1097,8 @@ class Item(models.Model):
             if stream.duration + offset < position:
                 offset += stream.duration
             else:
+                if not stream.file.is_video or not stream.file.info['video']:
+                    return None
                 position = position - offset
                 if not height:
                     height = stream.resolution