.art files?

Hi.

  I'm seeing some people start to use these for images and illustrations on 
their sites, but I can't find any info on what they are.  I can see the files 
in my browser, but my computer doesn't seem to know what to make of them when 
I save one.  I was wondering if anyone could tell me what they are, and maybe 
where I could find more info.  

Thanks.
-chris

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