Media files not protected by Multisite Privacy Plugin

Hi,

One of our users reported that media files are not protected by the Multisite Privacy Plugin. If one tries to access a site, for example:
http://sites.uci.edu/sylstestingsite/ then a login is required. However, if one is trying to access a file on the site, like http://sites.uci.edu/sylstestingsite/files/2012/08/anteater-350x250.jpg , then no login is required. The user found this out when sending a link to a PDF uploaded to the site.

How can we make all media files also protected?

Thank you,

Sylvia