MU / mu4e / short key

C-c RET f mml-attach-file (Attach with MIME)
C-c RET e mml-attach-external (Attach external file)
C-c RET b mml-attach-buffer   (Attach buffer; save to file)
S-return (mu4e-view-save-attach)
R reply
F forward
C compose
m move message other folder
u umark
U umark all
d to trash
D delete mark
? ! mark msg as unread/read