Fix article image peek if file extension wasn't all lower case.
Fix article image peek if file extension wasn't all lower case.

Noticed on first non-lead image on `enwiki > Freestyle swimming`.

josve05a/wikipedia-ios,josve05a/wikipedia-ios,wikimedia/apps-ios-wikipedia,montehurd/apps-ios-wikipedia,wikimedia/apps-ios-wikipedia,wikimedia/apps-ios-wikipedia,julienbodet/wikipedia-ios,montehurd/apps-ios-wikipedia,josve05a/wikipedia-ios,wikimedia/wikipedia-ios,montehurd/apps-ios-wikipedia,josve05a/wikipedia-ios,julienbodet/wikipedia-ios,wikimedia/apps-ios-wikipedia,josve05a/wikipedia-ios,montehurd/apps-ios-wikipedia,julienbodet/wikipedia-ios,wikimedia/apps-ios-wikipedia,wikimedia/wikipedia-ios,wikimedia/wikipedia-ios,josve05a/wikipedia-ios,montehurd/apps-ios-wikipedia,julienbodet/wikipedia-ios,josve05a/wikipedia-ios,wikimedia/apps-ios-wikipedia,montehurd/apps-ios-wikipedia,josve05a/wikipedia-ios,montehurd/apps-ios-wikipedia,julienbodet/wikipedia-ios,julienbodet/wikipedia-ios,wikimedia/wikipedia-ios,julienbodet/wikipedia-ios,wikimedia/apps-ios-wikipedia,wikimedia/wikipedia-ios,julienbodet/wikipedia-ios,wikimedia/apps-ios-wikipedia,wikimedia/wikipedia-ios,wikimedia/wikipedia-ios,montehurd/apps-ios-wikipedia