Steps to reproduce:
- Start a new topic at e.g. https://www.mediawiki.org/wiki/Talk:Flow
- Click </> to switch to wikitext mode and type in "[[File:Example.jpg|thumb]]"
- Click </> again to switch back to visual editing mode. (The image displays fine.)
- Click </> once again to switch to wikitext mode.
Result: The source has changed to "[[null|thumb|link=|alt=null|[[File:Example.jpg|link=|220x220px]]]]", and correspondingly the image displays with an extraneous "thumb|link=|alt=null|" next to it when switching back to the visual mode once again.