Total Commander

I've installed Smart Office and noticed that TC shows that application's icon for .DOCX/.XLSX files.

However standard file manager shows proper document icons (which look like paper sheets or albums with W or E letters, almost like on PC, perhaps taken from disabled app stubs for Word and Excel that suggest to install full apps).

Is there a way to get proper icons for documents in TC? If it does matter, I have Samsung Galaxy A320F with Android 8.

TC requests the icons from Android, so it's strange that you would get different icons. The icons are set by the app.
You could define a custom association for these file types, it allows you to pick your own icons.

Well, thank you for the hint, custom icon looks better, though application icon looks a bit different from file icon in other managers.

Perhaps Samsung file manager knows this format (though I doubt that it makes any presumptions), but other apps also show specific icons.
Here I made a screenshot showing how .docx and .xlsx files look in diferent apps: https://i.ibb.co/17Q72rx/Andro-Doc-Xlsx.gif
It looks like there is a set of icons for file extension somewhere, and other apps use icons from it (with different sizes).