Hi,

What about creating a File Storage plugin?
This would give the user one additional feature in the menu. Basically the user has a small little virtual harddisk. He can create new folders and upload files to his account and then always access them when he logs in to check his mail.
When a user composes a mail he can then easily select and attach his files through a drop down menu.

What do you think?

I think it would be a great addition! A good up-sell feature!