The document gives an absurd excuse for not supporting wildcard: However, writing a file requires knowledge of its format. Really? If an App saves some byte streams from other Apps that are determined by the user at runtime, it does not have to understand that binary. All it has to do is to write all bytes it receives to a file with an extension determined by the user.
Moreover, even the app knows the file format, it still may not decide the extension. The best example is the plain text format. We know there are so many different extensions that are essentially plain text. The app just has to write all the text provided by the user to the file and does not have to decide the extension on behalf of the user.
I think the real reason for not supporting wildcard is some other technical details. Whatever, I need this feature.