This is a very small and simple add-on. Just install the add-on and all external URLs and attachments in the attachment list open in a new browser tab.
Optionally you can define a CSS class, which will be added to every external URL and attachment link element in Confluence. This gives you the possibility to configure another display style for such links in the Confluence Stylesheets.
When you click on a link to an attachment within the page/blog post itself the default Confluence preview of the attachment appears. That does not change the current view, as soon as you close the preview it is restored. Hence, the external link plugin just affects attachment links in the attachment list belonging to the page/blog post.