This is a follow-up from a chat with DragonFlySixtySeven on IRC who suggested that there should be a way to prevent changing extensions accidentally when the user types the whole name such as (File selector) "Foobar,png" instead of "File:Foobar,png", to which MediaWiki throws an error because the extension change is disallowed.
My personal idea is to add another text field for the extension with the cursor being set to default to the page name.