I have certain custom data files for which I use the extension .gmy.
I associated this extension to an icon file named GMY.ico (created with VS 19); this contains icons for all the required formats, namely: 16x16, 32x32, 64x64, 48x48, 64x64, 96x96 - all 8 bit.
The association works fine as long as I use View | Medium size or Small size in File Explorer; when I select Large size, the icons show the same as for Medium size.
Any clue?