[MlMt] Add more specific buttons for attachment actions

[Thomas Eckhold]

> Currently there is „Quick Look“ and „Save“. I would love to 
> have „Open Attachment“ and „Save NAME …“

Both can be achieved with a right click (CRTL click) through the context 
menu. The „Save NAME …“ function can also be triggered by holding 
down the ALT key while clicking on the save button. To save all 
attachments you can use the shortcut ALT-CMD-SHIFT-A. And, of course, 
you can set up individual key bindings.

Regards
Thomas