wp-sanitize-file-name-upload.php

<?php

add_filter('sanitize_file_name', 'hwk_sanitize_file_name');
function hwk_sanitize_file_name($input){
    $path = pathinfo($input);
    $extension = (isset($path['extension']) && !empty($path['extension'])) ? $path['extension'] : '';
    $file = (!empty($extension)) ? preg_replace('/.' . $extension . '$/', '', $input) : $input;

    return sanitize_title(str_replace('_', '-', $file)) . ((!empty($extension)) ? '.' . $extension : '');
}