ASP文件下载.Net使用c#

我想让我的客户从我的网站下载文件。我试过这个代码:

string fName = Server.MapPath((new_exwork.FilePath));
FileInfo fi = new FileInfo(fName);
long sz = fi.Length;
Response.ClearContent();
Response.ContentType = MimeType(Path.GetExtension(fName));
Response.AddHeader("Content-Disposition", string.Format("attachment; filename = {0}", System.IO.Path.GetFileName(fName)));
Response.AddHeader("Content-Length", sz.ToString("F0"));
Response.TransmitFile(fName);
Response.End();

但是我得到了下面的异常:

无法计算表达式，因为代码已优化或本机帧位于调用堆栈的顶部。

MimeType函数

private static string MimeType(string Extension)
    {
        string mime = "application/octetstream";
        if (string.IsNullOrEmpty(Extension))
            return mime;
        string ext = Extension.ToLower();
        Microsoft.Win32.RegistryKey rk = Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(ext);
        if (rk != null && rk.GetValue("Content Type") != null)
            mime = rk.GetValue("Content Type").ToString();
        return mime;
    }