Comment 9 for bug 1958670

You could add a patch that rejects the filename and makes the user pick again I guess, but I don't personally think it makes much sense. Unless I'm misunderstanding things, once Pinta is patched so that it can open files regardless of their lack of extension, it shouldn't matter what they're actually saved as. Then you could remove the automatic appending of the extension to the filename which would fix this bug.

How this applies to Windows/Mac I'm unsure, there's a chance that the interaction with the native UI dialog's might make this a none issue, e.g on Windows, maybe you're always guaranteed to get an extension even if the user doesn't specify it manually? Unfortunately I've no clue, we'd have to test it I guess.