Why does /usr/share/Thunar/sendto/Bluetooth Device exist in a fresh install of Ubuntu Mate 18.04.1?
It’s part of the blueman package, that’s used by several desktops (including MATE) to manage Bluetooth.
Now, if you’re asking why there’s something specific to Thunar in a package that should be file manager-agnostic, I have no idea. Maybe Blueman was made with XFCE in mind in the beginning? Or it might be a packaging bug/oversight, probably in the Debian repos.
In any case, it’s normal that you have this, it’s part of a package preinstalled by Ubuntu MATE.
Thank you for the insight @terzag. That clears things up nicely.