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

@@ -375,30 +375,38 @@ 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))
       (org-entry-add-to-multivalued-property
        (point) org-attach-file-list-property basename))
     (let* ((attach-dir (org-attach-dir t))
-	   (fname (expand-file-name basename attach-dir)))
+           (fname (expand-file-name basename attach-dir)))
       (cond
-       ((eq method 'mv)	(rename-file file fname))
-       ((eq method 'cp)	(copy-file file fname))
+       ((eq method 'mv) (rename-file file fname))
+       ((eq method 'cp) (copy-file file fname))
        ((eq method 'ln) (add-name-to-file file fname))
        ((eq method 'lns) (make-symbolic-link file fname))
        ((eq method 'url) (url-copy-file file fname)))
       (when org-attach-commit
-	(org-attach-commit))
+        (org-attach-commit))
       (org-attach-tag)
       (cond ((eq org-attach-store-link-p 'attached)
-	     (org-attach-store-link fname))
-	    ((eq org-attach-store-link-p t)
-	     (org-attach-store-link file)))
+             (org-attach-store-link fname))
+            ((eq org-attach-store-link-p t)
+             (org-attach-store-link file)))
       (if visit-dir
-	  (dired attach-dir)
-	(message "File \"%s\" is now a task attachment." basename)))))
+          (dired attach-dir)
+        (message "File \"%s\" is now a task attachment." basename)))))
 
 (defun org-attach-attach-cp ()
   "Attach a file by copying it."