Attachments is a different topic, you cannot attach files simply by specifying some additonal command line switch. Moreover, you’ll have to encode attachments before mailing them. So, if you want to do that I’d suggest to you to check whether there’s some other command line mailing tool you can use for your purpose. Maybe there’s also some OSAX or Tool out there that allows sending mail from AppleScript under OS X, but unfortunately I don’t know such a tool.
You can also script the Mail application. Check out the scripts that iCal uses for ideas. The one problem I had was getting stylized text into the body of the message. Never worked. Resorted to copy paste using System Events.