Several people wrote to correct my inexact terminology in talking about URLs pointing at files available on the nets via FTP (and, at times in the future, Gopher or the World-Wide Web). URLs are not part of the HyperText Markup Language (HTML) as such, although they are somewhat related, given their joint usage on the World-Wide Web.
Fun Way to Send Attachments in Mail
If you're working in a file that you want to attach to a message in Apple Mail, you can transfer the file to Mail easily: From the title bar of the file's window, drag the little proxy icon to Mail's icon on the Dock. Your Mac will make Mail the active application and open a new outgoing message, with the file attached.
(If your icon won't drag, the file probably isn't saved.)