force your user to download that file though the
browser can open this file type.this requirement can
be accomplished using content-disposition header
in your page response.
the following example shows how to write a gif file
in response & force user only to download the file -
protected void Page_Load(object sender, EventArgs e)
{
Response.Clear();
Response.AddHeader("content-disposition", "attachment;filename=image.gif");
Response.ContentType = "image/GIF";
Response.WriteFile(Server.MapPath(@"~/image.gif"));
Response.End();
}