If your just providing a link to a file, the server should take care of this for you. You may need to add a mime type entry for a particular file. In
Tomcat, the mime types are in the global web.xml (the one in the conf directory).
If you're streaming the file to them from a database or location outside of the sections of your app that are accessible to browsers, you'll need to set the content type and the content disposition headers.
I have an example application that streams images from a directory under WEB-INF. It should have enough code to get you going.
http://simple.souther.us Look for SimpleStream.