Well, it's ill-formed, so what the UA does with it isn't really specified; cURL presumably treats it as a path relative to . because that's what the developer(s) decided that cURL should do. (For comparison, Firefox transforms it into file:///resolv.conf, then tries to find /resolv.conf on localhost and fails.)
I have never seen file:/ used before, so I can't really comment, but it seems invalid to me (and browsers auto-redirect it to ///).