While reporting T153862 I decided to check what is the situation with somewhat similar to the Commons-Datasets's Data namespace out-of-box File namespace, and I discovered that not only there's an edit button and no error message, but it is perfectly possible to create such pages (unless a wiki titleblacklisted them as Commons did): https://test.wikipedia.org/wiki/File:Example
I believe that such page creation should be prevented on engine level, similarly to how it is not possible to create extensionless Data namespace pages.
Perhaps it should be configurable with default to as requested. I can somewhat vaguely think on possible usages of such pages, but that is definitely not something to be desired by a majority.
TODO: Run a script/request to find all currently present 6th namespace extensionless pages.