<?php echo '<a href="templates/PATH TO YOUR K2 TEMPLATE-OVERRIDE/download.php?id='.$_GET['id'] = $this->item->id.'&title='.$_GET['title'] = $this->item->title.'" class="btn btn-default">download all files</a>'; ?>

<?php 		
    define( '_JEXEC', 1 );
    define( 'JPATH_BASE', realpath(dirname(__FILE__).'/../../../../../../' ));  
    require_once ( JPATH_BASE .'/includes/defines.php' );
    require_once ( JPATH_BASE .'/includes/framework.php' );
    $mainframe = JFactory::getApplication('site');
	$id = $_GET['id'];
	$title = $_GET['title'];

?>
<?php //database query to get the correct files
				$db_att=JFactory::getDBO();
				$db_att->setQuery("SELECT `filename`, `titleAttribute`, `itemID` FROM #__k2_attachments WHERE `itemID`= '{$id}' GROUP BY `titleAttribute`");
				$att_arr = $db_att->loadObjectList();

?>
<?php
				

// Prepare File
$file = tempnam("tmp", "zip");
$zip = new ZipArchive();
$zip->open($file, ZipArchive::CREATE);

// Stuff with content
foreach ($att_arr as $att_arrr) {
					$zip->addFile('../../../../../../media/k2/attachments/'.$att_arrr->filename, $title.'/'.$att_arrr->filename);
				}

// Close and send to users
$zip->close();
header('Content-Type: application/zip');
header('Content-Length: ' . filesize($file));
header('Content-Disposition: attachment; filename="'.$title.'.zip"');
readfile($file);
unlink($file); 
?>