Sure: When the client has created a hashsum of the file and sent it to the server, the server can respond with a challenge: "What's the SHA1 of the bytes betwee

Sure: When the client has created a hashsum of the file and sent it to the server, the server can respond with a challenge: "What's the SHA1 of the bytes between X and Y" (where X and Y are random numbers). This is something both parts can easily compute and the client must have the whole file in order to answer the challenge.