That will most likely not work.
I concur, though, that sendmail is not the right tool for the job. mail or mutt will serve you better, as they both support the -s option.
If you insist on sticking to sendmail, I'd try something like
Code:
(echo Subject: Your subject ; echo "" ; cat thefile ) | sendmail address@somewhere
Basically, when using sendmail, all headers (including subject) should already be present in the file. So one could even improve this further to:
Code:
(echo To: address@somewhere ; echo Subject: Your subject ; echo "" ; cat thefile ) | sendmail -t
Groetjes,
Kees-Jan