Proof of non-receipt of the file
Some partial idea: You want to avoid a situation where the user receives the file and claims they didn't. 

Bob adds some random padding to the file so that the user doesn't know the size. Bob sends part of the data and waits for a receipt, then sends more data and waits for a receipt. When the file is sent completely, the user doesn't know this. So refusing to send the receipt means they might never get the complete file. When Bob claims the whole file has been received, the user contacts Alice, who has a key to unlock the file.