Yes - you can add notes and files on task details view. To open task details, click on task item on tasks list or in calendar.

You will see all the tasks details, have an option to manage this task (Actions menu) and add a note.

After clicking on the "Share an update" field, a button to attach file will be shown. You can also attach a file by dragging it into this field from your desktop.

On this view you can also review all other notes added to this task.

Task creator and the person responsible will be notified when someone adds a new note to the task.