This feature was requested a while ago by the apps team, because file pages were not displayed in the app for files that exist on commons, not on the wiki. The feature works as follows:

1. Try to load get file page on-wiki
2. If 404 - try redirecting to commons
3. If 404 and it is already commons - throw 404.

The problem is that pages for files on commons do not really exist on-wiki, so Parsoid can't render it (example), also ChangeProp only gets notified for commons domain, but not for all the rest of the domains which the file is accessible through.

There're two approaches to fixing this I can think of:

1. Support rendering special-case file pages in Parsoid, stop redirecting, make a rule in ChangeProp that would rerender all domains for commons file change. The obvious downside of this approach is significantly higher load on Parsoid and Mobile-Content-Service for commons file uploads.
2. After adding support for language variants ( T122942 ) redirect to a specific variant of the page on commons. For this to work we can't use Accept headers for language variant specification, only some uri parameter, because obviously we can't specify which accept header to use after following the redirect and accept headers were the most likely way of representing language variants in the api.

@ssastry What would it take to support rendering file pages in Parsoid ?

Not sure if global user pages present the same problem (that they are parsed in different languages for different wikis). @Legoktm would surely know.

Yes, GlobalUserPages pass the same &uselang= parameter.

@Pchelolo, for now, I added an additional row in https://www.mediawiki.org/wiki/Parsoid/Known_differences_with_PHP_parser_output#Differences_because_of_implementation_differences_or_functionality_gaps .... Comments and thoughts welcome.

Change 392078 had a related patch set uploaded (by Niedzielski; owner: Sniedzielski):
[marvin@master] Update: show image on image File pages

https://gerrit.wikimedia.org/r/392078

Change 392078 merged by jenkins-bot:
[marvin@master] Update: show image on image File pages

https://gerrit.wikimedia.org/r/392078

Do we still need to work on this or it is resolved on the app side?

It seems like we're not *necessarily* saying that the redirect shouldn't happen, but that instead perhaps it ought to include a language parameter? T301372#8011652 discusses other places where the redirect ought to depend on the user language. See also T51097#7401266 regarding transclusion of file description pages from commons.