title: ETag usage with Amazon S3 Pre-signed URL
body: <p>Use case: we have client that can download pictures from the S3 storage using pre-signed URL and periodically refresh them. We don't need to download the picture again if it has not changed. 
  Problem: Can we use ETag and <code>if-none-match</code>header for solve our problem if pictures URL can change(e.g. after pre-signed URL expiration)?</p>