Until this is officially fixed, you can fix this problem by editing the magic-fields-2/admin/mf_upload.php file and replace this line:
$thumb = aux_image($filename,"w=150&h=120&zc=1",'image_alt');
with this one:
$thumb = ($file_type == 'image') ? aux_image($filename,"w=150&h=120&zc=1",'image_alt') : "";
Good luck!
