It is possible to attach files to mu4e messages using dired
(emacs), using the following steps (based on a post on
the mu-discuss mailing list by Stephen Eglen).
(add-hook 'dired-mode-hook 'turn-on-gnus-dired-mode)
Then, mark the file(s) in dired you would like to attach and press
C-c RET C-a, and you’ll be asked whether to attach them to an
existing message, or create a new one.