Unfortunately these are limitations of the Microsoft Windows 8 open/save file dialogs.
For open dialogs, we have to specify all the file extensions we support and it will only show files of those types. You can add/remove custom file extensions in the "File Types" area of the app's settings. That will let you get to various file extensions that aren't supported out of the box. But we can't tell the dialog to show *.* since that is prevented by Microsoft.
For save dialogs, I agree that it is annoying that it will append an extension but again, that is a limitation of the dialog that is provided to us. Adding custom file extensions in Settings will help with that too, assuming you pick a file type with that extension.
I wish there was more we could do but our hands are tied with these items. In addition, the open/save dialogs don't support files without an extension. For instance, an exception gets thrown if we try to display files like "*.".
We do have a couple helpful things though... as mentioned above, you can designate custom file extensions for any supported file type. And also, in our app Settings you can disable certain file types you don't use. That should make the Save As file types list shorter.