Vincent Massol, Simon Urli: Yes, thanks, it's fixed in 15.10.8. Sorry for the late reply, this Jira somehow didn't notify my about you comments, even though I'm reporter and listed as watcher.

I need to debug if it's just a misconfiguration in my user profile here, or a problem with my mail server, or a problem with the notification mails from this Jira.

Regarding the avatar images: Xwiki leaks the tmp file name in the mail's MIME structure - probably not an issue, but it reveals some (probably not too interesting) information about the server's file system layout:

X-TmpFile: /var/cache/tomcat9/Catalina/localhost/xwiki/xwiki-temp/mail/attachment14634703083894891407.tmp