add_filter( ‘wp_handle_upload_prefilter’, ‘custom_upload_name’ );
function custom_upload_name( $file )
{
if ( !$ext )
$ext = ltrim(strrchr($file[‘name’], ‘.’), ‘.’);
$file[‘name’] = date(“YmdHis”).rand(10,99).’.’.$ext;
return $file;
}

$file[‘name’] = rand(1000000000,9999999999).’.’.$ext;