API for developers. What Content-Type should I use?

I'm developing now API for developers feature, which gives users API access to some functions of our product.

All responses are in json format.

HTTP Content-Type header in our server's response is text/plain; charset=utf-8

Should I send one of this:

• application/json
• text/x-json
• text/json

instead of text/plain.

Or for API feature text/plain Content-Type is acceptable?

From stackoverflow

• application/json

• I'd use application/json as mentioned in the official JSON spec.