Any security aware website will be renaming uploaded files and hiding them in randomly named directories for this very reason,
But as Kulverstukas said, it is possible to figure it out by looking at other files that have been uploaded that you can locate on the main webpage, or if you are in the admin panel, you should be able to use the uploaded files on the website, for example put an image on a page, then go to the page and check the link of the image ect.