(define (file-name-extension file-name)
 (let ((extension-pos (find-in-string-from-end file-name #\.))
       (forward-slash-pos (find-in-string-from-end file-name #\/))
       (backward-slash-pos (find-in-string-from-end file-name #\\)))
  (cond ((and extension-pos forward-slash-pos (> extension-pos forward-slash-pos))
            (substring file-name (+ extension-pos 1) (string-length file-name)))
        ((and extension-pos forward-slash-pos (<= extension-pos forward-slash-pos))
            "")
        ((and extension-pos backward-slash-pos (> extension-pos backward-slash-pos))
             (substring file-name (+ extension-pos 1) (string-length file-name)))
        ((and extension-pos backward-slash-pos (<= extension-pos backward-slash-pos))
             "")
        (extension-pos (substring file-name (+ extension-pos 1) (string-length file-name)))
        (else ""))))