Custom CSV plugin [CSV Download Problem]

Hello,

I have written a custom plugin which will display some data from a particular table.
Main Admin View of this plugin

When I click on the ‘Export to CSV’ button the following window is opened

rather not giving me any option to download the converted CSV file.

I need that when I click on the ‘Export to CSV’ button it give me the csv file to download my computer.

Here is the code section in the plugin for converting mysql data to CSV.

// Get all the data from wp_newslette table
  $newsletter_result = mysql_query ("SELECT * FROM wp_newsletter WHERE newsletter = 1 ORDER BY nl_id DESC") or die (mysql_error());

  // Export all the data to CSV
  if ($_POST['submit'])
  {
    $out = '';
	$fields = @mysql_list_fields('iwords','wp_newsletter');
	$columns = @mysql_num_fields($fields);

	// Put the name of all the fields
	for ($i = 0; $i < $columns; $i++)
	{
	  $l=mysql_field_name($fields, $i);
	  $out .= '"'.$l.'",';
	}
	$out .="\n";

	// Add all the values in the table
	while ($l = mysql_fetch_array($newsletter_result))
	{
	  for ($i = 0; $i < $columns; $i++)
	  {
	    $out .='"'.$l["$i"].'",';
	  }
	  $out .="\n";
	}
	@header("Content-type: text/x-csv");
	@header("Content-Disposition: attachment; filename=filename.csv");
	echo $out;
	exit;
  }