It is important to note that a file sharing system cannot use an URL property describing which host stores the file. It could exist to denote the web site of the original manufacturer, but if such a property is used in other ways the system will be shut down, since it helps users find places to perform illegal downloads.
A file could be registered using appropriate metadata. Besides keywords or phrases describing the file, its size and SHA hash value are suitable properties to be registered. The system could also perform classification based on file types or MIME types. If files are given too much details it could be argued that these details endorse illegal searches and distribution. Storing hash values provides a way to check the validity of a file download from the Internet, which is a service many people and companies could benefit from.
The registrations of files could be performed using an XML encoded message which also makes it easy for an application to perform automatic registrations. Since such a simple scheme is an easy target for a malicious user, some kind of authentication is probably necessary. If the registrations are transmitted using HTTP a possible improvement would be to use HTTPS (HTTP over SSL, Secure Socket Layer) to establish a secure connection that requires authentication using certificates. This should lower the number of people that tries to harm the system. The best thing from a security standpoint is to require each user to register herself before receiving the necessary certificates.