func checkPostPolicy(formValues http.Header, postPolicyForm PostPolicyForm) error {
	// Check if policy document expiry date is still not reached
	if !postPolicyForm.Expiration.After(UTCNow()) {
		return fmt.Errorf("Invalid according to Policy: Policy expired")
	}
	// check all formValues appear in postPolicyForm or return error. #https://github.com/minio/minio/issues/17391