To help services understand the type of data they are receiving, they may want to know what type of content they're receiving, also known as MIME type. We can't always tell from the content itself what type it is, which is why we give this content type hint.
For a variety of reasons, this is not always obvious for JSON content as it can be used in a number of ways, and there are a few variations on it, like JSONP.
For plain JSON text, the content type is:
You might be confused by a number of different MIME types floating around the Internet, but to clear things up, you can find that this is the official content type as stated in RFC 4627.
Here are a few other MIME types that have also been used for JSON, but these are not the official content types, so beware:
If you're using JSONP (aka JSON with Padding), you'd use the content type:
For reference, you can find the full list of MIME types in the IANA Media Types page.