Use Dired to pick attachments for mu4e

dired4attachments.el

(defvar attachment-folder "~/Desktop/")
(defvar compose-buffer nil)

(with-eval-after-load 'mu4e
  (define-key mu4e-compose-mode-map (kbd "C-c C-a") 'lt/mml-attach-file))
(with-eval-after-load 'dired
  (define-key dired-mode-map (kbd "E") 'lt/process-marked-files-as-attachments))

(defun lt/mml-attach-file ()
  "Opens a Dired buffer to select attachments. The keybinding to send these back to the email draft is E."
  (interactive)
  (setq compose-buffer (buffer-name))
  (dired attachment-folder)
  (revert-buffer)
  (message "Press E to pick all marked files as attachments."))

(defun lt/process-marked-files-as-attachments ()
  "Processes all marked files in the Dired buffer as attachments for mu4e."
  (interactive)
  (if (dired-get-marked-files)
      (progn
	(let ((marked-files (dired-get-marked-files)))
	  (dired-unmark-all-marks)
	  (with-temp-buffer
	    (dolist (file marked-files)
	      (insert "<\#part filename=\"" file "\">\n" "<\#\/part>"))
	    (setq attachments (buffer-string))))
	(if compose-buffer
	    (lt/return-to-mu4e-draft)
	  (lt/create-new-mu4e-draft)))
    (message "No files selected.")))

(defun lt/return-to-mu4e-draft ()
  "Returns to the mu4e draft and inserts all attachments at point."
  (revert-buffer)
  (when compose-buffer 
    (switch-to-buffer compose-buffer)
    (save-excursion (goto-char (point-max)) 
		    (insert "\n\n" attachments)))
  (setq compose-buffer nil))

(defun lt/after-mu4e-compose (&rest _args)
  "Function to run after a new mu4e compose buffer is created. 
Accepts any number of arguments, but they are not used."
  (run-with-timer 0.1 nil
                  (lambda ()
                    (with-current-buffer (current-buffer)
                      (when (and (eq major-mode 'mu4e-compose-mode) 
                                 (not buffer-read-only))
			(save-excursion (goto-char (point-max)) 
					(insert "\n" attachments)))))))

(defun lt/create-new-mu4e-draft ()
  "Creates a new draft in mu4e and attaches the selected attachments."
  (advice-add 'mu4e-compose-new :after #'lt/after-mu4e-compose)
  (mu4e-compose-new)
  (advice-remove 'mu4e-compose-new #'lt/after-mu4e-compose)) 

I think attachments are supposed to be at the end of the email? Not sure about this. If so, I recommend L38 change to:

(save-excursion (goto-char (point-max))  (insert attachments)))

You are right. Also: save-excursion was made for this. Changed both inserts. Thanks!