Open button do not works properly in downloading with a linkbutton - asp.net

Question

There is a section in my site that users can download files. I want that As users push the download button and hit "Open" button instead of "save" button, the download get open with it's own program. For example a pdf file should not en with ms word. Now my problem is that, as a user open the file an XML extension will be added to the download URL!!
For example it should be:
www.mysite.com/inputfile/12.dox
but instead of that, it goes for
www.mysite.com/inputfile/12.dox.xml
Here is my cod:
protected void LinkButton1_Command(object sender, CommandEventArgs e)
{
int idx = Convert.ToInt32(e.CommandArgument);
string Manuscript = GridView2.DataKeys[idx].Value.ToString();
string fileName = System.IO.Path.GetFileName(Manuscript);
string contenttype = String.Empty;
string ext = Path.GetExtension(fileName);
Response.ClearContent();
Response.ClearHeaders();
switch (ext)
{
case ".doc":
contenttype = "application/msword";
break;
case ".docx":
contenttype = "application/msword";
break;
case ".xls":
contenttype = "application/msexcel";
break;
case ".xlsx":
contenttype = "application/msexcel";
break;
case ".jpg":
contenttype = "image/jpg";
break;
case ".png":
contenttype = "image/png";
break;
case ".gif":
contenttype = "image/gif";
break;
case ".pdf":
contenttype = "application/pdf";
break;
}
Response.ContentType = contenttype;
Response.AppendHeader("Content-Disposition", "attachment; filename=" + fileName);
Response.TransmitFile(Server.MapPath(Manuscript));
Response.End();
}