Chunk upload: item.uid can never be None so use it directly.

django_etebase/models.py

@@ -82,8 +82,7 @@ def chunk_directory_path(instance, filename):
     item = instance.item
     col = item.collection
     user_id = col.owner.id
-    item_uid = item.uid or 'main'
-    return Path('user_{}'.format(user_id), col.uid, item_uid, instance.uid)
+    return Path('user_{}'.format(user_id), col.uid, item.uid, instance.uid)
 
 
 class CollectionItemChunk(models.Model):