function new_filename($filename, $filename_raw) {
    global $post;
    $info = pathinfo($filename);
    $ext  = empty($info['extension']) ? '' : '.' . $info['extension'];
    $new = $post->post_title . $ext;
    // the if is to make sure the script goes into an indefinate loop
    if( $new != $filename_raw ) {
        $new = sanitize_file_name( $new );
    }
    return $new;
}
add_filter('sanitize_file_name', 'new_filename', 10, 2);