I'm having the same issue with Firefox Beta (118.0b9) on Android (11), the attach file button is completely unresponsive for me
Giving Firefox camera and microphone permissions is a working workaround for this issue. See https://bugzilla.mozilla.org/show_bug.cgi?id=1362919.
Twilight Sparkle
I've been using Firefox Nightly on a Pixel 4a.
Through both the PWA and a normal browser tab, pressing the "image upload" button does nothing. It stays highlighted, but does not respond. This is the only thing blocking the PWA from being properly usable for me.