Hello Thorsten

Thank you for your purchase.

I can't replicate this issue locally; the attachment is seen both when you edit the appointment from the panel, and when you click on it to expand:

Please provide me a temporary WP-admin login for your site where this happens, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course we do not provide login data to third party. 

You can write credentials here just check Private Reply so nobody can see them except us.

Unfortunately, at the moment, attachments can only be uploaded, and the plugin doesn't accept URLs.