With mailx you can do:
mailx -s "My Subject" -a ./mail_att.csv -S from=noreply@foo.com recipient@bar.com < ./mail_body.txt
This worked great on our GNU Linux servers, but unfortunately my dev environment is Mac OsX which only has a crummy old BSD version of mailx. Normally I use Coreutils to get better versions of unix commands than the Mac BSD ones, but mailx is not in Coreutils.
I found a solution from notpeter in an unrelated thread (https://serverfault.com/questions/196001/using-unix-mail-mailx-with-a-modern-mail-server-imap-instead-of-mbox-files) which was to download the Heirloom mailx OSX binary package from http://www.tramm.li/iWiki/HeirloomNotes.html.
It has a more featured mailx which can handle the above command syntax.
(Apologies for poor cross linking linking or attribution, I'm new to the site.)
