how can i prevent file from upload if it is exits (ajax file upload )

dear all

i have use ajax file upload and i want it not upload file if it exits

i try this in my code but i dont see any thing in lblmsg.text ? i want to prevent the upload and alert the client for that so pleasr how can i do that ?

var full_path = Server.MapPath((String.Format("~/Images/pt_img/{0}/{1}", Session["pt_id"], Path.GetFileName(e.FileName))));
            if (!File.Exists(full_path))
                {
                    AjaxFileUpload1.SaveAs(full_path);

                    var cn = new SqlConnection(cs);
                    var cm = new SqlCommand("add_pt_img", cn) { CommandType = CommandType.StoredProcedure };
                    cm.Parameters.Add("@pt_id", SqlDbType.Int).Value = Session["pt_id"].ToString();

                    // Save Image Name Only
                    cm.Parameters.Add("@img_name", SqlDbType.NVarChar, 150).Value = Path.GetFileName(e.FileName);

                    cn.Open();
                    cm.ExecuteScalar();
                    lblMsg.ForeColor = System.Drawing.Color.Yellow;
                    lblMsg.Text = "Patient Image Saved Successfully";
                    cn.Close();
                }
                else
                {
                    lblMsg.Text = "File exists!!!";
                    return;
                }