And it should be done such that the server treats everything the same. The incentive comes from deduped files counting less against storage quotas, and no time spent uploading the file. I'm just commenting on the general approach here, not the applicability to any particular type of service.

But your 'random salt' idea suffers from the attacker just generating all possible encryptions of the plaintext due to the small number possibilities. The "convergence key" is solely a security-parameter-length key that you can pass around to your friends so that your files will dedupe with theirs while not being susceptible to confirmation attacks by others.