If you look at the API of the DelegateTask you are passed in the listener, you will see that you can get a DelegateExecution via getExecution(), which in turn allows you to retrieve EngineServices via getEngineServices() - and if you look at that API, it allows you to retrieve various services, among them the TaskService.
Additionally, if you are developing in APS or another context with an annotation-based Spring dependency injection environment, you can simply have Spring inject the TaskService via @Autowired (provided you set your listener up to be a bean via @Component and access it via a delegate expression in your processes instead of a using it as a simple class-based listener).
Also note that, if you are working in APS, attachments are not handled via the TaskService. APS is a hack bolted on top of the core Activiti engine and it has its own RelatedContentService for dealing with attachments.
