While trying to resolve my other issue ()here) I was uninstalling and installing IrfanView recently from various sources and run into a number of weird issues with file associations:

1. Installation / copy from ZIP file fails completely, because ZIP file does not include .ini file. Instead of creating it during first run IrfanView claims about it being read-only.

2. When installing from App Store, it is not possible to set associations, because Extensions part of settings is disabled:

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It is disabled always, no matter, if you run IV as an admin or as an regular user.

3. When installing x64 version from .exe. file, Extensions part is always enabled, no matter, if you run IV as regular or as an admin even though it is said that you must run as admin in order to change extensions.

4. It is said that you should use Windows 10 Control Panel only, if you get some warnings. In fact you must use it always, because regular method to setting associations fails under Windows 10 1903. IrfanView claims that associations exists (shows all items as checked), but in fact no graphical file opens IV. You must use Control Panel in order to actually make associations.

This list is just to report the issues. I managed to resolve the problem (4th point), but wanted to report, in case other would like to check this.