;automatically register mode on .ext files
(defun sys-register-mode-with-file-extension 
  (mode extension)
  (setq auto-mode-alist
		(append auto-mode-alist
				'(((concat "\\." extension "$")  . css-mode)))))


;instead of doing something like this:
(setq auto-mode-alist (append auto-mode-alist '(("\\.css$"  . css-mode))))