Brief.
We have bug in download.php after release 3.0 has been issued on February, 27.
 
Description.
We have added check for downloading path whether it's local or not. Getting partial content is allowed only for local files now.
 
Solution.
Please download updated version of the file from here
http://www.viart.com/downloads/download-3.0.zip
Then extract file download.php into the root folder of your shop replacing existing one.