KeyConfig addon, and configure a new key: CTRL+ALT+M as binding, code: goDoCommand("cmd_attachFile") (note: no “;” at the end!), and voila, you can attach messages with CTRL+ALT+M!
And yes, there’s a similar extension called Nostalgy, which hides this behind Esc+A. I tried it, and it worked… if I kept Esc pressed while pressing A. That’s awkward. This is way better :)