If you delete a file in the Manage Files page using:
Manage Files > Bulk Action > Remove Files
File is NOT DELETED from server! I was able to find the deleted file still in the same place on the server, even though it has now disappeared from the Manage Files page. It also still remains in the database.
This means that eventually all the space on the server will be used up by deleted files and there is no way to get rid of the files without deleting ONE BY ONE!