I did a var dump of $_POST like so:
$post_type = $_POST['post_type'];
echo 'post type: ' . $post_type . '<br/>';
echo "<pre>";
var_dump($_POST);
echo "</pre>";
exit();
but it seems it never reaches here and throws a 404. The only thing that causes this problem is the hidden field. I have the plugin Debug Objects but am not sure how to catch the error here. A little advice would definitely help me debug and tell you the output.