Files can be uploaded by the employee and admin users, but it is also possible to give permission for managers to do this for the staff they line manage, too. To do this:
Go to Company Settings.
Open the Permissions tab.
Click 'Enable Editing'.
Tick 'Managers can view and add files for the staff they manage'.
Line managers will then be able to upload files to their reports' profiles and view any files that are shared with them.
NB: By default, only the person who uploads a file can see it in the system. If a user without permission to view a file opens the staff member's Files tab, they will not see the file there. Steps on how to share files with other users can be found here.