@jan-loseries,
I'm sorry but there is no way. Each attachment should be found in post contents, then inserted to the database to get ID, then the ID should be used to generate shortcode, then the file should be moved to correct folder and so on... There is no such tool doing that.
BTW you're showing the rendered HTML in the screenshot. All attachments of the addon are saved as shortcodes in the database and they are being rendered to the HTML on fly. If you go and check the database wpforo_posts table you'll see they look like this:
[ attach ] 234 [/ attach ]