It looks, that after one of changes that were deployed in Jan 4th, 2017 changes namespace aliases available on http://nn.wikipedia.org (see T154947).
Before these changes we have only one alias for File: Image, after them we have also Bilde.
Could someone confirm that it was intended and we need to fix test in Pywikibot? (@jhsoby ??)
My guess is that it was introduced in rOMWCc27a18eb84ee.
I'm not sure if it was intended or not, but it makes sense to have it as an alias, since that used to be the name of the file namespace in nowiki (Norwegian Bokmål Wikipedia), so having it as an alias for Nynorsk would make it easier to copy-paste articles for translation. However, I don't think that was introduced with the mentioned change, since that change didn't touch any Nynorsk wikis in any way that I can see.
In Pywikibot we run whole test suit on Travis after every merge. This build is the first one that have this test failing. Previous one didn't fail on that.
The change between this build in Pywikibot removed 2 broken unittests so it's not possible that it cause an issue there.
I'm not a PHP developer, but I believe this change was introduced in rMW or in rOMWC.
I will update Pywikibot test to accept this second alias and if You think, that's not worth to be rolback feel free to close it as invalid.
Ah, I think it might be related to this change, maybe: https://gerrit.wikimedia.org/r/#/c/329688/