[toomuchtodo]

So use the Dropbox as the golden copy/SHA reference, and fetch the file from peers over Bittorrent.

DB is always going to be able to be used like a resilient tracker/datasource, and DB devs can only put so many walls in the way of using it like that before it starts to hinder your "average" user.

Example: Get magnet hash, get data from Bittorrent swarm, and then fetch from both swarm and public copies available via Dropbox. Verify against Dropbox copy if necessary.