open attachment urls in a new tab

All you need to do is to add target='_blank' to the links leading to attachments.

Your search results template has something like this in it:

<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>

It should look something like this:

<a href="<?php the_permalink(); ?>" <?php if ($post->post_type == "attachment") echo "target='_blank'"; ?>><?php the_title(); ?></a>