[javascript] How do CORS and Access-Control-Allow-Headers work?

I'm trying to make CORS request POST from domain.com to a.domain.com.

My javascript looks like this

$('#fileupload').fileupload({
  xhrFields: {
    withCredentials: true
  },
  dataType: 'json',
  url: $('#fileupload').data('path'),
  singleFileUploads: true,
  add: function(e, data){
    data.submit();
  }
});

At first I see the OPTIONS route being called like so:

Request URL: https://a.domain.com/some/route
Request Method:OPTIONS
Status Code:200 OK

OPTIONS REQUEST:

Access-Control-Request-Headers:origin, content-type, accept
Access-Control-Request-Method:POST
Host:a.domain.com
Origin:http://domain.com:3000
Referer:http://domain.com:3000/home

OPTIONS RESPONSE

Access-Control-Allow-Credentials:true
Access-Control-Allow-Methods:POST
Access-Control-Allow-Origin:http://domain.com:3000
Connection:keep-alive
Content-Length:0
Content-Type:text/html;charset=utf-8

That request comes back with a 200 like stated. On my server, I have the same route with POST method and this is what I get in return after the OPTIONS

Request URL:https://a.domain.com/some/route

POST REQUEST

Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryjwr5Pk7WBcfzMdbO
Origin:http://domain.com:3000
Referer:http://domain.com:3000/home

and the POST request gets canceled/fails.

My question is, do I need to have the access-control-allow-origin on the POST controller as well?

I have a cookie for authorization that has domain .domain.com that cookie got sent across once in a request and it's not being sent now. Any idea why that would happen?