Hi goplaa,

Here i have created sample that fullfill your requirement.

Here i have used the below article to insert binary data to database.

Read and Write BLOB Data to SQL Server database using C# and VB.Net

HTML

<div>
    <asp:DropDownList runat="server" ID="ddlCountries" OnSelectedIndexChanged="ddComapanyFilter_SelectedIndexChanged"
        AutoPostBack="true">
        <asp:ListItem Text="Test 1" Value="516" />
        <asp:ListItem Text="Test 2" Value="517" />
        <asp:ListItem Text="Test 3" Value="518" />
        <asp:ListItem Text="Test 4" Value="519" />
    </asp:DropDownList>
</div>

Code

protected void ddComapanyFilter_SelectedIndexChanged(object sender, EventArgs e)
{
    string id = ddlCountries.SelectedItem.Value;
    byte[] bytes = null;
    string contentType = string.Empty;
    string name = string.Empty;
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        con.Open();
        SqlCommand cmd = new SqlCommand("SELECT * FROM tblFiles WHERE Id = @Id", con);
        cmd.Parameters.AddWithValue("@Id", id);
        using (SqlDataReader reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                bytes = (byte[])reader["Data"];
                contentType = reader["ContentType"].ToString();
                name = reader["Name"].ToString();
            }
        }
        con.Close();
    }
    Response.Buffer = true;
    Response.Charset = "";
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    Response.ContentType = contentType;
    // If you want to open in browser then comment the below line of code "Response.AddHeader("content-disposition", "attachment;filename=" + name);".
    Response.AddHeader("content-disposition", "attachment;filename=" + name);
    Response.BinaryWrite(bytes);
    Response.Flush();
    Response.End();
}

Screenshot