While browsing the archive for Xbox.com there was a file titled 'darwin' downloaded with an unspecified file type, I tried opening in Windows Photo Viewer and VLC Media File.
It was on a reveal page for Xbox One. Mysteriously, I still can't find out any info on this anywhere on Xbox Forums, so it must be weird. This was near 6 days before it's realized.
On mega.nz, the decryption key is the part of the URL that lets other people access the files. If you go back and right click the file, you can select to copy the link with the decryption key on the end.