i found the workaround. it turns out if you don't have a media player installed on your phone (i got VLC and it remedied this) it will only offer to open it in whatsapp, files is gone! once i installed VLC on my phone (I'm sure you can get any media player I'm not endorsing) and airdropped my audiobook i was offered to open it in WhatsApp, VLC and Files became an option again! Don't know why WhatsApp have manage to take over incoming audio drops (haven't tried video), but for files to disappear as an option unless you get another app is crazy. needs fixing apple !