I do not know about mail but I used Mutt, through that you can attach file.
it's ease to use. for more information use mutt at console.
About Sendmail, you can not create attachment using sendmail. it's a MTA
Creating e-mail messages, including adding attachments or signatures, is the function of a mail user agent (MUA). Some popular MUAs include mutt, elm, exmh, Netscape, Eudora and Pine.