Hey Guys,
AppleScripted Signatures are broken in Apple Mail since Yosemite (if I remember correctly), and Apple in their infinite wisdom has chosen NOT to fix them. (Bad Apple!)
So Gabe's solution is probably the best one at this time.
NOTE – Keep in mind that Keyboard Maestro's Send Mail Message action can only send one file – it cannot manage multiple files.
This would appear to be fine for Martin's task at this time, but should that change please see post #2 in this topic:
-Chris