I think this is an Windows issue, after one day of evaluating the problem, I created a linux based docker development xwiki instance, where everything seems to work normaly. But if so, we need a hint that it will probably not work in Windows envionments (with the Windows Demo instance).
This message was sent by Atlassian Jira (v9.3.0#930000-sha1:287aeb6)
If image attachments aren't displayed, see this article.