Earlier quoted context omitted.
Thanks for the info. Let me see if I understand this correctly. Browsers don't send the anchor tag (ie: with GET requests). FF Send takes advantage of this by using the anchor tag to store the key for decryption. That is kinda novel. You still need to trust the upload client to not leak the key, but I see that you've written a CLI version. Interesting! Thanks for the response.
It's not a new idea, the megaupload successor first did it as far as I can remember
More seriously, did they do anything to fix this obvious design flaw? If they want to fish a key they can just serve you a modified JS file and retrieve the key. Unless of course you chose to audit the JS served every time you browse the website.