'category_name'
will not work. You are making use of a custom taxonomy, not build-in categories. It seems from this that you have a misunderstanding about taxonomies. Please take your time and see this post on this particular subject
For custom taxonomies, use a tax_query
to retrieve the posts that you need. Here is an example:
$query_images_args = array(
'post_type' => 'attachment',
'post_mime_type' =>'image',
'post_status' => 'inherit',
'posts_per_page' => -1,
'tax_query' => array(
array(
'taxonomy' => 'attachment_category',
'field' => 'slug',
'terms' => 'SLUG OF YOUR TERM',
),
),
);
$query_images = new WP_Query( $query_images_args );