mu4e: custom compose-resend that includes in-line attachments

custom-compose-resend.el

(defun my/mu4e-compose-resend ()
  (interactive)
  (advice-add 'mu4e~compose-handler :after #'my/mu4e-compose-resend-processing)
  (mu4e-compose 'resend))

(defun my/mu4e-compose-resend-processing (&rest args)
  ;; Make sure we are in a compose buffer
  
  (unless (derived-mode-p 'mu4e-compose-mode)
    (error "Must be in a mu4e-compose buffer."))
  
  ;; remove all headers preceding the "From:" line

  (goto-char (point-min))
  (re-search-forward "^From:")
  (delete-region (point-min) (line-beginning-position))

  ;; Is this a multipart message?

  (let ((case-fold-search t))
    (when (re-search-forward "^content-type: multipart" nil :noerror)
      (mime-to-mml)
      
      ;; remove the first line of the message (mime multipart separator)

      (let ((end-pos (point))
            (start-pos (message-goto-body)))
        (delete-region start-pos end-pos))))
  
  ;; Remove unneeded headers
  (message-remove-header "Date")
  (message-remove-header "Message-ID")
  (message-remove-header "X-TUID")    

  (message-goto-body)
  (advice-remove 'mu4e~compose-handler #'my/mu4e-compose-resend-processing))
  
(define-key mu4e-headers-mode-map (kbd "S") #'my/mu4e-compose-resend)
(define-key mu4e-view-mode-map (kbd "S") #'my/mu4e-compose-resend)