That link should do for our purposes here.

The fact that it is an 'x-' mimetype means it has not been approved at
any level.  There might be an in progress application to the mimetype
registry, but if so the web site doesn't mention it anywhere obvious.

I'm not sure about the filetype problem, but I'm guessing amp isn't the
only mimetype that will have this issue going forward, so we probably need
to come up with a solution.  You don't need support from the mimetypes
module to create or manipulate emails using the content-type, though,
so it isn't a blocker on that side.

That lightning thing is *seriously* hokey :(