(defun add-auto-mode-re (re mode)
  (assq-delete-all re auto-mode-alist)
  (setq auto-mode-alist (cons (cons re mode) auto-mode-alist)))

(add-auto-mode-re "\\.sx.\\'" 'archive-mode)

(defun jk-xml-prettify ()
  "Put each tag on a separate line and indent."
  (interactive)
  (save-excursion
    (while (search-forward "><" nil t)
      (replace-match ">
<" nil t))
    (indent-region (point-min) (point-max) nil)))

(defun jk-xml-zip-hook ()
  (let ((suffixes '("\\.xml\\'")))
    (while (and suffixes (not (string-match (car suffixes) buffer-file-name)))
      (setq suffixes (cdr suffixes)))
  (if suffixes
      (jk-xml-prettify))))

(add-hook 'archive-extract-hooks 'jk-xml-zip-hook)