wp_get_attachment_url filter won’t accept two arguments

I think you’re missing the number of callback input arguments, so try this one instead:

add_filter( 'wp_get_attachment_url','mfeu_filter_attachment_url', 10, 2 );
                                                                  |  |
                                                 Priority    _____|  |
                                      Number of arguments    ________|

The default is 10, 1, so you’re currently only passing on the $url and not the second $post_id argument.