Use message/external-body MIME type for registering 'redirect' binaries

Here's an idea for handling "redirect" binaries. I think it's a nice compromise between the empty-binary-with-a-special-property and creating a whole new type of thing.

Plus, I don't think we abuse the MIME type too much: https://tools.ietf.org/html/rfc2017

https://www.pivotaltracker.com/story/show/81770804