Add views / models / etc etc to support StockItem attachment

InvenTree/stock/models.py

@@ -6,6 +6,8 @@ Stock database model definitions
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+import os
+
 from django.utils.translation import gettext_lazy as _
 from django.core.exceptions import ValidationError
 from django.urls import reverse