It's good that Dropbox can help recover from Ransomware. I believe the service could do more to prevent Dropbox contamination and alert users. Consider having a unique-per-user file in each Dropbox which would be checked for its expected value on every upload, and which if changed would prevent further uploads and alert user. Would need to be a filename that hackers couldn't easily predict or discover and avoid changing. Given the goal, perhaps better ways could be invented? If they couldn't zap my Dropbox, my most important files could be safer.