Java Programming

This is probably more simple than I think it is, but I'm having trouble reading files back correctly (outside the program) after I've programmatically made the zip file.

I'm trying to zip up selected contents of a database by getting the item, and then writing the contents of the database entry out to the zip file through a byte stream. This all works fine, but all the entries in the zip file are "files" with no extension, so they don't open properly, if at all. Is there a way to save file types correctly in a zip file based on content? The database doesn't preserve filenames, all items are referred to by unique ID's. Any help is much appreciated.