drupal7下载私有附件的问题

Question

function hook_file_download($uri) {
// Check if the file is controlled by the current module.
if (!file_prepare_directory($uri)) {
$uri = FALSE;
}
if (strpos(file_uri_target($uri), variable_get('user_picture_path', 'pictures') . '/picture-') === 0) {
if (!user_access('access user profiles')) {
// Access to the file is denied.
return -1;
}
else {
$info = image_get_info($uri);
return array('Content-Type' => $info['mime_type']);
}
}
}
这个方法怎么才能被调用，来验证请求，
还望大神指教