1. Login to Amazon S3 Console: https://console.aws.amazon.com/s3/home.
  2. Locate your bucket, click on the same. On clicking, the right pane will show properties for that bucket. Expand Permissions, Click "Edit CORS Configuration" and update CORS to allow force.com.
  3. Add following XML snippet
  4. <?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
   <CORSRule>
      <AllowedOrigin>*.force.com</AllowedOrigin>
      <AllowedOrigin>*.visualforce.com</AllowedOrigin>
      <AllowedMethod>PUT</AllowedMethod>
      <AllowedMethod>POST</AllowedMethod>
      <AllowedMethod>DELETE</AllowedMethod>
      <AllowedMethod>GET</AllowedMethod>
      <AllowedHeader>*</AllowedHeader>
   </CORSRule>
</CORSConfiguration>


  5. For more info click here