Thanks for your help.
This is what I came up with:
attachment.php in /pro-child folder
<?php
// =============================================================================
// ATTACHMENT.PHP
// -----------------------------------------------------------------------------
// Handles output of attachment page.
// =============================================================================
?>
<?php x_get_view( x_get_stack(), 'wp', 'attachment' ); ?>
and wp-attachment.php in /pro-child/framework/views/ethos/ in folder
<?php
// =============================================================================
// VIEWS/ETHOS/WP-ATTACHMENT.PHP
// -----------------------------------------------------------------------------
// Single post output for Ethos.
// =============================================================================
$fullwidth = get_post_meta( get_the_ID(), '_x_post_layout', true );
?>
<?php get_header(); ?>
<div class="x-container max width main">
<div class="offset cf">
<div class="<?php x_main_content_class(); ?>" role="main">
<?php while ( have_posts() ) : the_post(); ?>
<?php x_get_view( 'ethos', 'content', get_post_format() ); ?>
<?php echo wp_get_attachment_image( get_the_ID(), 'full', "", array( "class" => "img-responsive" ) ); ?>
<?php echo do_shortcode('[vc_widget_sidebar sidebar_id="ups-sidebar-attachment-page-footer"]'); ?>
<?php endwhile; ?>
</div>
</div>
</div>
<?php get_footer(); ?>
I didn’t understand the point of content-attachment.php in global folder, so removed it all together.