The thumbnail at the permalinked-post URL
https://www.mediawiki.org/w/index.php?title=Topic:Sdcuk73nuv7i3cmf&topic_showPostId=sdcuk78ao1pvbb8n#flow-post-sdcuk78ao1pvbb8n
is linking to
https://www.mediawiki.org/w/File:Example.jpg
(note the /w/ rather than /wiki/)
There is nothing strange about the wikitext used. It should just work normally.
It works normally at https://www.mediawiki.org/wiki/Topic:Sdcuk73nuv7i3cmf