Including files and attachments in expiration items in automatic notifications is done at the category level. Each category has an option to automatically include these files in the notifications.

To enable including files in notifications first click on Your Account on the top menu and then on Settings.

Then click on Categories on the left menu under the "Manage" heading.

Then click on any category from the list you're wanting to include files for.

In the new or edit screen, click on the Advanced tab and tick the check box 'Include Attachments on Email Notifications':

Then click Save to apply the changes/create the category.