ASP.NET

//pageLoad of GetFile.aspx
//call like: http://www.mywebsite.com/GetFile.aspx?MyID=123
private void Page_Load(object sender, System.EventArgs e)
{
    // Put user code to initialize the page here
    int MyID = Convert.ToInt32(Request.QueryString["MyID"]);

    SqlConnection connection = new 
        SqlConnection (@"server=INDIA\INDIA;database=iSense;uid=sa;pwd=india");
    connection.Open ();
    try
    {
        SqlCommand command = new
          SqlCommand("select myFile, myFileMime from MyTable " +
                          "where MyID = @MyID", connection);
        command.Parameters.AddWithValue("@MyID", MyID);

        byte[] myFile = null;
        string myMimeType = "";
        SqlDataReader reader = command.ExecuteReader();
        reader.Read();
        myFile = reader.GetBytes(0, 0, myFile, 0, int.MaxValue); //column 0
        myMimeType = reader.GetString(1); //column 1
    }
    finally
    {
        connection.Close();
    }
        
    Response.Clear();
    Response.ContentType = myMimeType;
                
    // this line replaces the html with the content you send it.
    Response.BinaryWrite(myFile);  

    Response.End();
        
}

in MyHtmlPage.html
<img src="GetFile.aspx?MyID=12345" width="100" height="100">
where file id 12345 is an image

in myHtmlPage.html
<a href="GetFile.aspx?MyID=12345">Download file</a>
where file id 12345 is a file to download.

DropDownList1.Attributes.Add("onchange", "YourDisableScript(...);");