protected function initExtensions()
 {
     if (!$this->package->hasExtensions()) {
         return;
     }
     $this->writer->startElement('files');
     foreach ($this->package->getExtensions() as $ext) {
         $this->writer->startElement('file');
         $this->writeAttributeIfValueNotEmpty('type', $ext->getType());
         $this->writeAttributeIfValueNotEmpty('id', $ext->getName());
         $this->writeAttributeIfValueNotEmpty('group', $ext->getGroup());
         $this->writeAttributeIfValueNotEmpty('client', $ext->getClient());
         $this->writer->text($ext->getFile()->getName());
         $this->writer->endElement();
     }
     $this->writer->endElement();
 }