This link will only work one time.
The fragment in the link, created by the sender, contains data to initiate a key-exchange protocol.
This web-page runs a real-time, client-side, key-exchange protocol, communicating directly with the sender.
The sender pushes a message encrypted to you with the session key (derived from the key-exchange).
The sender's message contains:
This web page downloads the encrypted file and decrypts it with the file key all client side.
Note: the sender will only participate in the key-exchange protocol exactly once. First come first serve.
After the first receiver initiates the key-exchange protocol (the first time this page is opened), this link is useless to anybody else that finds it.