(define (handle-attach message auth-fid-value bind-fid! reply! error!)
  (dump-message-fid 'Tattach message auth-fid-value)
  (let ((root (filesystem-root filesystem)))
   (bind-fid! root)
   (reply! (list (file-qid root)))))

...and the value passed to bind-fid! comes back to later handlers as a fid-value like so:

(define (handle-open message fid-value reply! error!)
  (dump-message-fid 'Topen message fid-value)
  ;; FIXME: Check permissions, and have a hashtable in the filesystem
  ;; that maps a (session, file id) pair to an open-state in which to store
  ;; anything we need, and expose it to extra per-file handlers.
  (reply! (list (file-qid fid-value) +block-size+)))