Предоставляет ли Amazon S3 пользовательские заголовки? Или мне не повезло?
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST, GET, OPTIONS
Access-Control-Allow-Headers: X-SOMETHING
Access-Control-Max-Age: 1728000
Пример
var request = new XMLHttpRequest();
var url = 'http://example.s3.amazonaws.com/templates/welcome.html';
function callOtherDomain(){
request.open('GET', url, true);
request.withCredentials = "true";
request.onreadystatechange = handler;
request.send();
}