How to do meta_query for attachments?

I went with custom taxonomy instead of custom meta field and it works great (although I am still not sure why meta_query didn’t work)

I have saved custom taxonomy term for each attachment

wp_set_object_terms($post->ID, $slug, 'image_category' );

And custom query args are

$args = array(
        'post_type' => 'attachment',
        'posts_per_page' => 44,
        'post_status' => 'inherit',
        'post_parent' => null,
        'tax_query' = array(
            array(
                'taxonomy'     => 'image_category',
                'field'   => 'slug',
                'terms' => $cat
                )
            )
        );