Right now, the only clear way to add a File for use with an Event is to add it inside of the Event. Once added, it can be used by other Events and can be viewed and deleted from the Files area under Admin. However, there's no apparent way to upload a File outside of doing it through an Event. Additionally, it's not terribly clear if there's a limitation to the number of Files that can be uploaded for use, or if there's a size limitation of any kind.