For a further example of using access-control-request-headers: Content-Type see this MDN example: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Headers#examples

Remember, the server must be configured to properly reply to the preflight OPTIONS request with access-control-allow-headers. You'll need to monitor the HTTP exchanges carefully to make sure this is happening, and reconfigure Apache or whatever other web server you're using.