Implementation of hook_file_download

A hook for not displaying file upload (node & comment) for private content :

/**
 * Implementation of hook_file_download().
 */
function private_file_download($file) {
  $file = file_create_path($file);
  $result = db_query("SELECT f.* FROM {files} f WHERE filepath = '%s'", $file);
  if ($file = db_fetch_object($result)) {
	$node = node_load($file->nid);
	if ($node->private == 1) {
		if (node_access('view', $node) == FALSE) {
			return -1;
		}
  	}
  }
}