Currently nothing. But I've only mentioned this extension to a small technical audience, so I wouldn't expect a flood of spam. In practice, this needs some sort of "local captcha" that doesn't involve 3rd parties, i.e. the storage server returns a puzzle (like guess what's shown on this 3d image) and if solved correctly, it results in a proper hash that's easy to verify. A simpler, but uglier solution is to ask the user to produce a hash with N leading zeros.