Thank you for the fast reply.
After checking, it looks like everything works fine for new attachments.
What I had noticed is that after I turned on encryption and shared the synchronisation folder using SyncThing, when importing on a new device the notes were encrypted properly but the attachments were lost.
Since then (no new configuration change), new attachments are shared correctly.
Not sure it is worth a new specific topic, but maybe something to double check in the future.
Thanks again for your help and for this very useful piece of software.