Restricting access to uploaded files

Hello
Im building a site where the user can upload files. 
If I, in my browser, enter the complete url to the upload folder, like this: http://localhost:3599/XYXYServer/files/someFolder
I can reach all uploaded files.
My ASP.NET application will make sure that a user only can reach his/her own files.
So, I need to prevent the above way to look at uploaded files.
Any ideas?
 
Thanks...
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