Currently, it appears as if only the uploader of a file can change the view permissions of or delete a file; it does not appear that an even an administrator has the ability to do this.
If a user inadvertently or maliciously uploads something they shouldn't have, and either promptly goes on vacation (in the former case) or ignores my request to remove the file (in the latter case), my only recourse appears to locate where the file is physically stored and remove it by hand, which is exceptionally inconvenient.