We use blat to do this all the time in our environment. I use it as well to connect to Gmail with Stunnel. Here's the params to send a file
blat -to [email protected] -server smtp.example.com -f [email protected] -subject "subject" -body "body" -attach c:\temp\file.txt
Or you can put that file in as the body
blat c:\temp\file.txt -to [email protected] -server smtp.example.com -f [email protected] -subject "subject"