Ah, it only works without tagget="_blank". Thank you very much!
The problem I also mentioned was that (in Windows 10, not with android) I can click on the following link and see the picture. But the rest of the notes content isn't available anymore and I only get out of this situation, if I restart joplin.
# Markup
[Markup Download](http://bbz-dormagen.de/home/images/2017/2017-11/PlakatBeratungstag2018.jpg)
# HTML
<a href="http://bbz-dormagen.de/home/images/2017/2017-11/PlakatBeratungstag2018.jpg">HTML Download</a>
When I click on Markup Download, a new broswer tab is opened.
When I click on HTML Download, the image is shown in the HTML layout pane and I can’t get rid of it unless I restart Joplin.