@spacresx was on the right track. You need to do one of two things:
1. Create an index file (index.php, index.html, index.htm). This file will show up as the default if a full file name is not given.
2. Turn on auto index. This is usually not what you want and it is a mild level security issue. Create a file named .htaccess and put in the file
Keep in mind that doing #2 will give your users the ability to see any file name you upload, and possibly, the contents of that file. Allowing your users to do this is usually a bad idea. It's safer to create index.html and then make some basic links <a href="file.gif">file.gif</a> rather than expose your innerworkings.