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 or gnus-dired-attach, and you’ll be asked whether to attach them
to an existing message, or create a new one.