Grayscale image in wordpress

Functions.php

<?php
   add_filter('update_attached_file', 'convertToGrayscale');
   function convertToGrayscale($location,$id)
   {   
      
      $info = getimagesize($location);
      
      switch ($info['mime'])
      {
         case 'image/jpeg':
            $image = imagecreatefromjpeg($location);
            break;
         case 'image/gif':
            $image = imagecreatefromgif($location);
            break;
         case 'image/png':
            $image = imagecreatefrompng($location);
            break;
    }
    
    
    imagefilter($image, IMG_FILTER_GRAYSCALE);

      
      switch ($info['mime'])
      {
         case 'image/jpeg':
            imagejpeg($image,$location,100);
            break;
         case 'image/gif':
            imagegif($image,$location,100);
            break;
         case 'image/png':
            imagepng($image,$location);
            break;
      }
      
      imagedestroy($image);
   }
?>