public function isValid()
 {
     $public_id = $this->resource_type == 'raw' ? $this->filename : $this->public_id;
     $expected_signature = Cloudinary::apiSignRequest(array('public_id' => $public_id, 'version' => $this->version), Cloudinary::configGet('api_secret'));
     return $this->signature == $expected_signature;
 }