When I Google around searching for any code snippets I might need for inspiration (or plain copy-paste), I see a lot of different use of MIME type for JSON in examples and existing code out there.
Examples of types found with a quick JSON google search:
You could of course argue that the reason for this is that the browsers do have a bit of history when it comes to supporting “standards”, but there is only one correct media type for JSON:
You can find this registered at IANA (Internet Assigned Numbers Authority), take a look here.
What about JSONP?
If you find this useful or in any way disagree, holla at me!