It is only possible to add an attachment to an issue in the Description field via drag&drop. It is still not possible to add an attachment with a comment or via the Attechments-Section that the customer can see / access.

We have seen the issue JSD-610 and followed the description where it's said:
_"You can add files and images to any issue in your service desk project. When working on an issue, simply drag and drop a file onto the issue, or select More > Attach files. You will then have the option to add a comment with more information about the attachment, and then share the file and comment with your customer or with your internal team only.
When adding or editing a comment, you can also select to add attachments. In this case, you'll see wiki markup added to the comment field. As soon as you share your comment, you'll see the file preview."_

So this seems to be a bug as it's said it should work.