protected void Page_Load(object sender,EventArgs e)

 {

      if(!IsPostBack)

     {

         System.Io.FileStream myFileStream = new System.Io.FileStream(Server.MapPath("",System.Io.FileMode.Open));

         long FileSize;

         FileSize = myFileStream.Length;

 　　　　 byte[] Buffer = new byte[(int)FileSize];

         myFileStream.Read(Buffer,,(int)myFileStream.Length);

         myFileStream.Close();

         Response.Clear();

         Response.ClearHeaders();

         Response.Content();

         Response.ContentType = "image/png";

         Response.AddHeader("content-disposition","attachment:filename="+"文件名.png",System.Text.Encoding.UTF8)

         Response.BinaryWrite(Buffer );

     }

 }