lisp/org-attach.el (org-attach-attach): Add dired-dwim-target-directory

Use case: have two windows open side-by-side. One has an Org-mode
file, the other - a Dired buffer with a file we want to attach.

With this change, and user's `dired-dwim-target' setting, the prompt
for file to attach will start in the Dired buffer's directory.

lisp/org-attach.el

@@ -375,7 +375,15 @@ Only do this when `org-attach-store-link-p' is non-nil."
 If VISIT-DIR is non-nil, visit the directory with dired.
 METHOD may be `cp', `mv', `ln', `lns' or `url' default taken from
 `org-attach-method'."
-  (interactive "fFile to keep as an attachment: \nP")
+  (interactive
+   (list
+    (read-file-name "File to keep as an attachment:"
+                    (or (progn
+                          (require 'dired-aux)
+                          (dired-dwim-target-directory))
+                        default-directory))
+    current-prefix-arg
+    nil))
   (setq method (or method org-attach-method))
   (let ((basename (file-name-nondirectory file)))
     (when (and org-attach-file-list-property (not org-attach-inherited))