All the files uploaded to the site are displayed under Files.
The usage of disk space can be viewed on the settings front page.
The disk space depends on your site plan.
When the storage limit is reached, an additional 1GB is automatically added to your site (with the price of 2€ per month). Additionally, a notification e-mail is sent out to all Administrator users regarding reaching the site storage limit.
The Administrator of the site can select who has what rights concerning files under Settings > Administration > Permission sets.