WebClient downloads xls file with content “session expired”

[nikel]

I'm trying to download some excel files using C#. The original files have no extension and when I check they contain only this string:

<?xml version="1.0" encoding="ISO-8859-9"?><err timeout="1">Oturum Süresi Doldu.</err>

(It's session expired in English)

Here's my code:

using (var client = new WebClient())
{
    client.DownloadFile(downloadLink,
            Application.StartupPath + "\\xls\\" + linkList[1] + ".xls");
}

The source site was created with asp. How can I download these files properly?