what is namespace for extension string

Actually iam copy the google code. I am getting error in extensionstring. once check my code
private void UploadFileUsingASPNET(FileUpload fileupload)
{
try
{
// check if file has been selected
if (FileUploadSingle.HasFile)
{
string path = Server.MapPath("~/MultipFileUpload/");
string fileName = Path.GetFileName(FileUploadSingle.PostedFile.FileName);
// check for the valid file
extensionstring fileExtension = Path.GetExtension(fileName).ToLower();
if (fileExtension.Equals(".gif") || fileExtension.Equals(".jpg"))
{
// now save the file to the disk
FileUploadSingle.SaveAs(path + fileName);
lblMessage.Text += "File : <b>" + fileName + "</b> uploaded successfully ! <br />";
}
else
{
lblError.Text = "Only .gif and .jpg files are allowed.";
}
}
else
{
lblError.Text = "Please select a valid file.";
}
}
catch (Exception ee)
{
lblError.Text = ee.Message;
}

}