CFBundleTypeIconFiles or UTTypeIconFiles?

Dear Experts,

I'm attempting to make a custom icon appear in the iOS Files app, etc., for my file type. I've found a couple of bits of documentation for Info.plist keys:

https://developer.apple.com/documentation/bundleresources/information_property_list/utexportedtypedeclarations/uttypeiconfiles describes UTTypeIconFiles, to be included in UTImportedTypeDeclarations. This documentation is very sparse!

Older document https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html#//apple_ref/doc/uid/TP40009249-SW9 describes CFBundleTypeIconFiles, to be included in CFBundleDocumentTypes.

Which of these should I be using? (Both? Neither?)

Nothing I've tried so far has worked. I wonder if I need to, for example, power-cycle to make the Files app pick up the new icons.

Also, in Xcode, I've found the Imported Type Identifiers section of the Info settings which has a box labelled "Add imported type identifiers here" - but clicking + and choosing a file does nothing; the box remains empty. Anyone else have that problem?

What size should the icons be? The older document suggests some rather small sizes, e.g. 22x29; the newer doc says nothing.

Suggestions anyone?