if sha256Sum, ok := r.Form[xhttp.AmzContentSha256]; ok && len(sha256Sum) > 0 {
			contentSHA256, err = hex.DecodeString(sha256Sum[0])
			if err != nil {
				return ErrContentSHA256Mismatch
			}
		}
	} else if _, ok := r.Header[xhttp.AmzContentSha256]; !skipSHA256 && ok {
		contentSHA256, err = hex.DecodeString(r.Header.Get(xhttp.AmzContentSha256))
		if err != nil || len(contentSHA256) == 0 {
			return ErrContentSHA256Mismatch