upload-dir.php

<?php
add_filter('upload_dir', 'cgg_upload_dir');
function cgg_upload_dir($dir)
{
    // xxx Lots of $_REQUEST usage in here, not a great idea.

    // Are we where we want to be?
    if (!isset($_REQUEST['action']) || 'upload-attachment' !== $_REQUEST['action']) {
        return $dir;
    }

    // post types match up?
    if (!isset($_REQUEST['post_id']) || 'ml_resource' !== get_post_type($_REQUEST['post_id'])) {
        return $dir;
    }

    $exts = apply_filters('ml_resource_whitelist_ext', array('jpe?g', 'png', 'gif', 'bmp'));

    // let images to do their own thing
    if (!isset($_REQUEST['name']) || preg_match('/(' . implode('|', $exts) .')$/ui', $_REQUEST['name'])) {
        return $dir;
    }

    // modify the path and url for other files.
    $resources = apply_filters('ml_resource_directory', 'resources');
    $dir['path'] = path_join($dir['basedir'], $resources);
    $dir['url'] = path_join($dir['baseurl'], $resources);

    return $dir;
}