Web Mail currently limits you to adding three file attachments to a message, using the three entry boxes at the bottom of the compose view. We are working on updating that feature to remove this limitation. In the meantime, user Jack Sweitzer has come up with a very clever way to work around the limit, that doesn't require using zip files (as I've suggested before).
Someone asked, on Dec 5, 2007: "Can't you remove the 3 attachment limit on the "write message" dialog box? This is a real nuisance. I often need to include more attachments!"
Until this is changed, I found a way around it. First, post your three attachments in the boxes provided, and then save to Draft. Next, re-open your draft message . . . your three attachments will show above as attachments, and your three boxes for adding attachments will be empty, allowing you to add three more. Continue until all your attachments are added. I recently did it for 10. The only problem is, if the sequence of attachments is important, you'll need to add them in reverse order; that is, say you have 9 items to attach: start with #7-9, then 4-6, then 1-3.