Make files visible to external users
Every file has a "visible to external users"-setting; The little eye indicates if the file is visible to external users:
You can change the "visible to external users"-setting by clicking on the pencil.
Tip: In your personal settings > defaults for new items, you can configure the "visible to external users" default for new files you upload.
User permission
You can choose to hide all files for external users.
The permissions panel can be found when you go to system settings > external users > your user.