[ticket/14285] Use private cache-control for atttachments

PHPBB3-14285

phpBB/phpbb/storage/controller/attachment.php

@@ -271,6 +271,16 @@ class attachment extends controller
 		return parent::handle($attachment['physical_filename']);
 	}
 
+	/**
+	 * {@inheritdoc}
+	 */
+	protected function prepare($file)
+	{
+		parent::prepare($file);
+
+		$this->response->setPivate();
+	}
+
 	/**
 	 * Handles authentication when downloading attachments from a post or topic
 	 *