It was "on the roadmap" as of 4 years ago…

https://github.com/signalapp/Signal-iOS/issues/905

…but there's been no apparent progress.

On some core usability & functionality matters, Signal appears to be a really, really, lethargic development team, both before and after the arrival in early 2018 of $50 million in funding.

Which previously made sense when the file system was basically untouchable by end users. But with files/ios13 it makes no sense at all.

Ah, my bad. It works on Android which is all I've ever used it on.