Which JSON content type do I use?

Of course, the correct MIME media type for JSON is application/json, but it's necessary to realize what type of data is expected in your application.

For example, I use Java Ext GWT and the server response must go as text/html but contains JSON data.

Client side, Ext GWT form listener

uploadForm.getForm().addListener(new FormListenerAdapter()
{
    @Override
    public void onActionFailed(Form form, int httpStatus, String responseText) 
    {
        MessageBox.alert("Error");
    }

    @Override
    public void onActionComplete(Form form, int httpStatus, String responseText) 
    {
        MessageBox.alert("Success");
    }
});

In case of using application/json response type, the browser suggests me to save the file.

Server side source code snippet using Spring MVC

return new AbstractUrlBasedView() 
{
    @SuppressWarnings("unchecked")
    @Override
    protected void renderMergedOutputModel(Map model, HttpServletRequest request,
                                           HttpServletResponse response) throws Exception 
    {
        response.setContentType("text/html");
        response.getWriter().write(json);
    }
};

The right content type for JSON is application/json UNLESS you're using JSONP, also known as JSON with Padding, which is actually JavaScript and so the right content type would be application/javascript.

The correct answer is:

Content-Type: application/json