Thanks again, all.
I tried another download - this time GentiumBookPlus.woff from
https://en.bestfonts.pro/font/gentium-book-plus
(not zipped)
Crashed again.
Notepad++ shows the first four characters are "wOFF".
It appears that everyone who has (gratefully!) replied has .woff as an installed "known" file extension.
I'd like to get that nailed down. I don't know what forum would be appropriate for that. Maybe I could try Dell.
Dell Vostro 7620 x64-based, purchased new June 2023. Windows 11 Pro 10.0.22621