I’ve often felt confused with the difference between the Content-type and Accept Headers that are passed along with a request to an API. I’ve used a fake Json REST API called JsonPlaceHolder to show the Json request and response with a Post request.

Accept Header tells the API that it is expecting the response in the specified media type e.g. application/json or application/xml.
Accept: application/json

And Content-Type tells the API about the media type of the request being sent in the request body e.g. application/json.
Content-Type: application/json

Both headers are sent along with the call to the API from the Client. Below I’m using the Postman Client to test the scenario for json request.
For the scenario, to test the response in XML format, the API should support the format provided in the Accept Header.

The 2nd image above shows the Json body being passed in the Post request for Creating the resource.

For a standard HTML Post request, the Content-Type Header will be one of the Standard types:

1. application/x-www-form-urlencoded for simple ASCII text
2. multipart/form-data for file upload support or non-ASCII text