When adding any category to a redirect page in the File: namespace, the category name does not display in the page's bottom "Categories" box, although the page itself does get added to the category.
In the given example URL, the page is in [[Category:Test category]], and when you go to the category page "Category:Test category" it is there, but on the redirect page itself the category is not showing. ("Category:Test category" is a non-existent category but that doesn't matter, it's the same for any category).
To reproduce, simply: Go to any file redirect using [[Special:RandomRedirect/File]], and try adding a category to the page, either using Hotcat or by manually editing the page.
This was supposed to be fixed along with [[bugzilla:23851]] in the new 1.17 release but I guess it was overlooked.