How to properly use mailx to attach a file (not replace content)?

I am using

$ mailx -V
mailx (GNU Mailutils) 3.5
Copyright (C) 2007-2017 Free Software Foundation, inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
$ echo test | mailx -s Testmessage user -A /tmp/test.png

in Debian buster. However, the in e-mail client (such as Thunderbird, Apple Mail etc.) I only see the attached picture displayed; text does not show up!
If I look at the message source, the message is there:

--385425503-1612411894=:25669
Content-Type: text/plain; charset=UTF-8
Content-Disposition: attachment
Content-Transfer-Encoding: 8bit
Content-ID: <20210204051134.25669.1@localhost>

test

--385425503-1612411894=:25669
Content-Type: application/octet-stream; name="test.png"
Content-Disposition: attachment; filename="/tmp/test.png"
Content-Transfer-Encoding: base64
Content-ID: <20210204051134.25669.1@localhost>

iVBORw0KGgoAAAANSUhEUgAABVYAAAK2CAYAAACl2sceAAAgAElEQVR4nOzdeXRUhcH/f9t6WtvH
[...]

I played with --alternative, --content-type and -M but could not find an option that works.