mautrix-whatsapp places a file in /tmp/ for file transfers. However,
signald is running with PrivateTemp=true. This uses systemd’s feature
that allows a unit to share a namespace with another one and places
mautrix-signal in the same namespace as signald, so they share their
private tmp directory.
Sadly, they are so interconnected, that it is not easily possible to
migrate it in smaller steps. It should be refactored to make them more
modularised and independent of each other.
Simon Bruder