mu4e: use mu4e-attachment-dir for attaching, too

Use mu4e-attachment-dir also for attaching (and not just for extracting)
attachments; the default (current draft message dir) is not very useful.

mu4e/mu4e-compose.el

@@ -477,6 +477,11 @@ buffers; lets remap its faces so it uses the ones for mu4e."
           (visual-line-mode t))
       (setq mml-enable-flowed nil))
 
+    ;; set the attachment dir to something more reasonable than the draft
+    ;; directory.
+    (setq default-directory (mu4e~get-attachment-dir))
+
+
     (let ((keymap (lookup-key message-mode-map [menu-bar text])))
       (when keymap
         (define-key-after