Sublime Text adds a convenient 'Open with Sublime Text' entry in the context menu of every file, but - I guess since Windows 10 - this gives the 'How do you want to open this file?' dialog, if there is no default program associated with that extension. Since it's just a normal registry command (instead of a shell extension), I guess this behaviour is not limited to Sublime Text.
Command:
C:\Program Files\Sublime Text 3\sublime_text.exe "%1" Key:
HKEY_CLASSES_ROOT*\shell\Open with Sublime Text\command
Windows Explorer (and Notepad's Open dialog) just opens the file in Sublime like expected.
I tried on a few filetypes, including a randomly typed extension, without problems here.
Please run my FileTypeDiag tool on one of the types you're seeing the problem with and attach the results so we can see the full registry details.
(The command's exe path should really be in quotes, since it contains spaces, but you can get away without the quotes as long as no C:\Program, or "C:\Program Files\Sublime" or "Sublime Text" folders exist to confuse things. If it works in Explorer and Notepad then it's OK, but you might want to add the quotes to avoid surprises.)
I think I know how it works. When you install a new application on Windows 10 compatible with a specific extension, Windows always re-asks what the default application should be when opening such a file. So now I get the dialog for .css and .s files, because I installed Visual Studio recently. css.txt (233 KB)