Create 'New Email with Attachment (via web-based Gmail)' option in Finder?
Try mutt, the command line email client that you can use with shell scripting: 

This page seems like a excellent resource:
https://wiki.archlinux.org/index.php/mutt

New email from command line:

mutt -s "Subject" somejoeorjane@someserver.com -a somefile &lt; /tmp/sometext.txt


Now you just need to modify that to go to Drafts instead of send (I have not tried this). Anyway, you can make an Automator service that runs one of these shell commands at mutt, then make a keyboard shortcut for that service.