I am using Sharecenter to download onto a PC,
My colleagues have a similar problem on other Windows PCs - we have tried both Firefox and Internet Explorer with the same result. We have tried with different file types and it always adds the prefix and suffix.
There is a post regarding the same problem logged for a DNS320L - but no resolution
http://forums.dlink.com/index.php?topic=55351.msg213803#msg213803
and for a DNS345
http://forums.dlink.com/index.php?topic=55465.msg215249#msg215249