Seems like I got this working now!
This is how I made my filemanager.class.php file to look like:
class FileManager {
private $_directories;
private $_mode;
private $_manage_cache;
private $_manage_dir;
private $_manage_root;
private $_recently_uploaded = array();
private $_sub_dir;
private $_sendfile = false;
//private $_max_upload_image_size = 350000;
private $_max_upload_image_size = 6291456; // 6MB
public $form_fields = false;
Thanks!