text: Title: Add attachment to order_placed email
  Body: I need to add an attachment to the `order_placed` email. I see the function I would ideally hook into. The `Checkout.mixins.OrderPlacementMixin` there is a function called `send_order_placed_email`.
  
  So would I fork the whole Checkout app? And add my own `mixins.py` and override the `OrderPlacementMixin`?