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//////////////////////////////////////////////////////////////
///////// ADD MEDIA PREFIX TO ATTACHMENT PERMALINKS
//////////////////////////////////////////////////////////////
add_filter( 'wp_unique_post_slug', 'media_premalink_prefix', 10, 6 );
function media_premalink_prefix( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
    if ( $post_type == 'attachment') {  
        $prefix = 'media';
        if ( strripos($slug, $prefix) == 0 ) {
            $slug = $prefix . $slug;
        }
    }
    return $slug;
}