Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
</head>
<body>
<?php
if(empty($_POST['forma']))
{?>
<form id="form1" name="form1" method="post" action="mostrar_lista.php">
<p>
<?php
include "conexion.php";
conectar();
$con=mysql_query("select *from sc");
echo "<select name='forma'id='forma' style='text-transform:uppercase'>";
echo "<option value='0'>Elige..</option>";
while($registro=mysql_fetch_row($con))
{
echo "<option value='".$registro[0]."'>".$registro[1]."</option>";
}
echo "</select>";
?>
</p>
<p>
<label>
<input type="submit" name="enviar" value="Enviar" />
</label>
</p>
</form>
<?php }else{
$forma=$_POST['forma'].".xls";
}?>
</body>
</html>