Comment by Xantier

I've built my own, called KEIBIDROP :D but did not release the mobile apps let

https://github.com/KeibiSoft/KeibiDrop

With Linux it's very easy as long as you're okay going offline - just make an ad hoc wifi network, have the other device connect to it, then you can do whatever (Localsend, host a local webserver on the linux machine, whatever.)

I used to use Nitroshare, but Localsend has supplanted it.

Localsend and KDE Connect