Edited Unassigned: AjaxFileUploadHandler.axd request to server does not include cookie with sessionid [27756]

All other requests to the server include the cookie, which has the sessionid and lets the load balancer know which server to stay on, except for the post to the AjaxFileUploadHandler.axd. I have put two screenshots from fiddler so that you can better understand what I mean. In one Request Header, you can clearly see that it includes the cookie, which includes the sessionid. In the other which includes the POST to the AjaxFileUploadHandler.axd, you can see that it does not contain any cookie. How can I make it include the cookie in the POST request? I am willing to modify the code of the AjaxControlToolkit to get this to work.

POST /AjaxFileUploadHandler.axd?contextKey={DA8BEDC8-B952-4d5d-8CC2-59FE922E2923}&fileId=3AE2923C-8010-0953-2918-628F9725CD0F&fileName=Chrysanthemum.jpg&usePoll=true&storeToAzure=false&acn= HTTP/1.1