public function getFileSource()
 {
     if (!$this->id) {
         return null;
     }
     $cacheFile = $this->_getCacheFile();
     if (!file_exists($cacheFile)) {
         $s3 = new Kwf_Util_Aws_S3();
         $r = $s3->get_object(Kwf_Config::getValue('aws.uploadsBucket'), $this->id, array('fileDownload' => $cacheFile));
         if (!$r->isOk()) {
             $body = file_get_contents($cacheFile);
             unlink($cacheFile);
             throw new Kwf_Exception($body);
         }
     }
     return $cacheFile;
 }