Create Custom Attachment Template That is Processed from Plugin Folder?

Filter attachment_template and return your custom path. For details, see wp-includes/template.php, function get_query_template().

Sample code:

add_filter( 'attachment_template', function( $template )
{
    global $post;

    // inspect $post object to determine the correct template
    // and change the file path, then:
    return $template;
});