Redmine

Hello!
Right now I'm developing plugin that adds an extra tab for attaching files. I've had a couple of questions about Redmine:
How work association with table "attachments"? In controller "Files" there is :include :attachments line while model "Attachments" only :belongs_to :container. It seems that this ":container" somehow work with associations. Can you explain how?
I'm really confused, so any help will be appreciated!