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

@escowles 
@ajs6f 

https://www.pivotaltracker.com/story/show/81770804
I assume "message/external-body" is a completely fabricated mediatype. In which case, should it not be in either the "vendor tree" (vnd) or "personal tree" (prs)?
http://en.wikipedia.org/wiki/Internet_media_type#Vendor_tree