Just read the reappeared sourcecode (assuming it works as advertised): The hash is an SHA256 of pure 4MB blocks in the input file. They add no message type information which could prevent mixups between Dropbox-deduplication hashes and hashes computed for other purposes.

The following dropship file was assembled using only shasum, ls and vi:

         {"blocks": ["f3f754a5dcd93f271ad013a5ee84f495a36da84f152e0a1fec4646345b0c10d6"], "name": "ostseestrand.jpg", "size": 514779}

Could someone who has never shared files with me verify that it indeed produces a picture of a beach?