Do you want all of your planner files to have a TXT extension so that
they’ll be searched by tools such as Google Desktop or Apple Spotlight?

Use the ‘rename’ tool to add .txt extensions to all of your files, or
use the following code to rename all of your planner files from a
running Planner setup:

(defun my-rename-planner-files ()
  "Rename all my planner files to .txt if they don't have that extension yet."
  (interactive)
  (mapcar
   (lambda (file)
     (unless (string-match "\\.txt$" (cdr file))
       (rename-file (cdr file) (concat (cdr file) ".txt"))
       (message "%s" (cdr file))))
   (planner-file-alist))
  (with-planner
    (emacs-wiki-refresh-file-alist)))

Then add this to your ~/.emacs before you load Planner:

(setq emacs-wiki-ignored-extensions-regexp "\\.txt")

(defadvice emacs-wiki-find-file (around extension activate)
  "Open the Emacs Wiki page WIKI by name.
If COMMAND is non-nil, it is the function used to visit the file.
If DIRECTORY is non-nil, it is the directory in which the Wiki
page will be created if it does not already exist."
  (unless (interactive-p)
    (setq wiki (cons wiki
                     (cdr (assoc wiki (emacs-wiki-file-alist))))))
  ;; At this point, `wiki' is (GIVEN-PAGE FOUND-FILE).
  (if (cdr wiki)
      (let ((buffer (funcall (or command 'find-file) (cdr wiki))))
        (if (= (prefix-numeric-value current-prefix-arg) 16)
            (with-current-buffer buffer
              (set (make-variable-buffer-local 'emacs-wiki-directories)
                   (cons (file-name-directory (cdr wiki))
                         emacs-wiki-directories))
              (set (make-variable-buffer-local 'emacs-wiki-file-alist)
                   nil)))
        buffer)
    (let* ((dirname (or directory
                        (emacs-wiki-maybe t)
                        (car emacs-wiki-directories)))
           (filename (expand-file-name (car wiki) dirname)))
      (unless (file-exists-p dirname)
        (make-directory dirname t))
      (funcall (or command 'find-file) (concat filename ".txt")))))