OK so I strongly suspect a problem a lack of mime type support in Android < 10. @Arturo and @Luca, what do you think about disabling the "install from file" feature on Android < 10. It looks like Android 9 isn't supported at all according to https://en.wikipedia.org/wiki/Android_version_history
Bug 1868940 Comment 5 Edit History
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OK so I strongly suspect a lack of mime type support in Android < 10. @Arturo and @Luca, what do you think about disabling the "install from file" feature on Android < 10. It looks like Android 9 isn't supported at all according to https://en.wikipedia.org/wiki/Android_version_history