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This used to be simple to do in XP (example guide), but I can't figure out how to do it in Win7.
Currently extensionless files open by default in notepad (yes I have the show file extensions enabled).
When I try to use the Open With dialog to associate the Scite editor with such files, the Always use the selected program option is greyed out.
I've tried the XP trick of adding a . (dot) key in HKEY_CLASSES_ROOT. If I add the key OpenWithList to it, and then and a subkey scite.exe, the OpenWith context menu will show scite. But it still doesn't allow me to set it as the default viewer. Perhaps there's another registry trick I don't know of.
Note that simply renaming a file isn't always possible, for example I might be working on a foreign source code repository which has a posix-style extensionless README file.