Dropbox blocks sharing of copyrighted files
Dropbox is able to identify and block the sharing of copyrighted material without actually accessing personal files. The system used to achieve this has been used for many years and allows Dropbox to restrict the sharing of pre-selected files.
Source: http://techcrunch.com/2014/03/30/how-dropbox-knows-when-youre-sharing-copyrighted-stuff-without-actually-looking-at-your-stuff/?ncid=tcdaily