CharlieRB and w32sh, Thank you! I particularly appreciate w32sh's link to FileTypeDiag, which seems to be a useful tool I hadn't been aware of.
What DOES work is the text in the MS link, but which is not precise enough; the corresponding example is outright misplaced or even wrong.
When you only want a new icon for a new file type (i.e. one that is not yet associated to some application), you do the following in the registry if the suffix / file type is .x and the full path of the (.exe, .ico or other) file containing the icon is c:\y.ico
In HKEY_CLASSES_ROOT
.x (i.e. you create a new key, which you leave blank, don't leave out the dot, it's part of the key name)
.x\DefaultIcon (you create a new subkey DefaultIcon within your new key .x)
with the following value:
(Standard) (type: REG_SZ) c:\y.ico (no double quotes ("") needed)
After either shutting down and restarting your pc down or simply a user logout followed by user login (a mere user change will be as insufficient as a simple "refresh the registry"), you will then see your .x files, in your file manager, together with their new icon from within the y file. (The same is true after changing the icon without changing the icon file name / location, i.e. when you replace the content of the icon file: Then again the new icon will only be visible after a restart or a user-logout-login.)
Cheers!
DefaultIcon has to be defined at Prog ID level. This guide should help. How to Change the Default Icon for a File Type in Windows?
– w32sh – 2016-09-20T17:16:34.767Don't put (SOLVED) in your title, check the checkmark next to the answer that solved the problem or make your own answer and accept it if you solved it yourself. – Scott Chamberlain – 2016-09-20T20:26:49.430
Pretty harsh that this question has negative points. It's a pretty basic problem a newbie will have with trying this stuff on Windows. The Windows extract icon interface(s) is/are still busted in so many ways and documentation on how it 'really' works is so lacking from Microsoft. In fact, it's fair to say that the 'proper' way to use it - really doesn't exist except for a few explicit ways that Explorer uses internally. They should have dumped it in favour of the newer IThumbnailProvider interface to create icons when they added that with Vista and ditched this broken mess as legacy. – mysticcoder – 2017-09-11T06:54:21.550