On the System Info page, which you can access by choosing Configuration > System Info from the Admin main menu, the Filestore usage info seems to be incorrect.
This is a known issue. Similar information can however be obtained from the Troubleshooting Tool, which was introduced with WebCenter 12.1.2.
Symptoms
Other information on the System Info page looks correct, but the Filestore usage info section shows 0 for MB used.
In fact, FileStore space is used, since files have been uploaded to WebCenter.
Workaround
This is a known issue, which will be addressed in later versions of WebCenter.
If you are using WebCenter 12.1.2 or newer, follow these steps to see the FileStore usage info:
As an Admin user, choose Configuration > Troubleshooting Tool. You will see that several tests are running, wait for the tests to complete.
Check the information displayed at the test File Store Disk Space [WCR121_TST_320].