Download file from SQL server database using ajax and web service in ASP.NET

I've tried to download a specific file from a SQL Server database using Ajax and a web service and a code is working without errors and still can't download the file.Here is the code

My HTML

<pre><input id="btn_download" type="button" value="download_att" />

my ajax function to read id for file retrive

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<pre lang="Javascript"><pre lang="Javascript">$('#btn_download').click(function () {
                id = $('#Tid').val();
                $.ajax({
                    url: 'WebService1.asmx/DownloadFile',
                    method: 'POST',
                    contentType: 'application/json;charset=utf-8',
                    data: '{id:' + JSON.stringify(id) + '}',
                    success: function () {
                        alert("s");
                    },
                    error: function (err) {
                        alert(err);
                    }
                });
            });

my webservice

[ScriptMethod]
[WebMethod]
public void DownloadFile(int id )
{
    byte[] bytes;

    string fileName, contentType;
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["TesterConnectionString1"].ConnectionString);

    using (SqlCommand cmd = new SqlCommand())
    {
        cmd.CommandText = "select image from attach where Id=@Id";
        cmd.Parameters.AddWithValue("@Id", id);
        cmd.Connection = con;

        con.Open();

        using (SqlDataReader sdr = cmd.ExecuteReader())
        {
            sdr.Read();
            bytes = (byte[])sdr["image"];
            contentType = sdr["id"].ToString();
            fileName = sdr["id"].ToString();
        }

        con.Close();
    }


    httpContext.Current.Response.Clear();
    httpContext.Current.Response.Buffer = true;
    httpContext.Current.Response.Charset = "";
    httpContext.Current.Response.Cache.SetCacheability(HttpCacheability.NoCache);
    httpContext.Current.Response.ContentType = contentType;
    httpContext.Current.Response.AppendHeader("Content-Disposition", "attachment; filename=" + fileName);
    httpContext.Current.Response.BinaryWrite(bytes);
    httpContext.Current.Response.Flush();
    httpContext.Current.Response.End();
}