Are you still in the old email format or the new one? To tell, look at the URL in the address bar. If it says Zimbra you are in the old one, if it says Appsuite you are in the new one. If the old one, when in the Compose page click on "Attach" in the toolbar and navigate to the item you want. If the new, when in the Compose page click on "Attachments" under the Subject field and then navigate to the item you want.