Finally, let’s define an attachment action. As mentioned, attachment-action functions receive 2 arguments, the message and the attachment number to use.
The following example action counts the number of lines in an attachment, and defines n as its shortcut key (the n is prefixed to the description).
(defun count-lines-in-attachment (msg attachnum) "Count the number of lines in an attachment." (mu4e-view-pipe-attachment msg attachnum "wc -l")) ;; defining 'n' as the shortcut (add-to-list 'mu4e-view-attachment-actions '("ncount lines" . count-lines-in-attachment) t)