User blog comment:DaNASCAT/Technical Update: June 19, 2017/@comment-24014422-20170619205119/@comment-5211495-20170620094912

Not possible. The Multimedia Viewer extension requires MediaWiki 1.25+. Fandom's version is 1.19.24. And to avoid compatibility issues Fandom is not likely to upgrade the MediaWiki base.

I think the trick is to span the current image viewer fullscreen.