The file is not stored in the back-end of the site. It is stored in the publicly accessible images directory. Anything stored there is public information. If you would block access to this directory, you would not be able to include any images on your site.
If you want to prevent search engines from indexing files in your images directory you have to edit the robots.txt file. Google "robots.txt" for more information.
Note: robots.txt is ignored if your site is in a subdirectory. This file is only respected when it's located in the domain's root. Perhaps this is why your images did get indexed.
Nicholas K. Dionysopoulos
Lead Developer and Director
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