Not (currently) with the Send Mail action. A couple ways to do it:
1) Call CDONTS (I believe this comes w/ IIS) from a Run Script action:
http://www.devasp.com/Samples/Mail3.asp
2) Call blat (
http://www.blat.net/) from a Run Program action:
blat "drive:\path\to\filename.htm" -server <server> -f <from address> -t <to address> -s "subject" -u <user> -pw <password>