Per rule #3, this seems to be a general home computing question and not centered around self-hosting. Please consider adding details to clarify how this involves self-hosting.
Syncthing, Resilio Sync, or one of those browser based p2p file sends e.g. https://file.pizza or similar.
If both p2p ends know how to use torrents then creating a simple torrent to share to the other peer would work fine. But that requires slightly more IT competence especially if someone needs to open a port forward (ideally you would make sure you have your own port forwarded so the other party doesn't have to worry about this).
If you're doing this more than once it might be worth setting up a simple server e.g. HFS is a nice open source/free HTTP file server, been a while since I used it but it still seems to be active https://www.rejetto.com/hfs/
Most likely because it is a self-contained web server executable? That sort of software would seem suspicious and similar to other sorts of virus/malware.
Granted I have not used it in a while but I'd be surprised if it were true, their forums and github would be full of reports of malware.
Check out QuickDAV. I think it’s exactly what you’re looking for. If you’re going across the internet, you’d have to forward a port from your router. Otherwise, if you’re on the same network, it’s really simple.
Can we talk about how utterly absurd it is that there isn’t an obvious answer to this question yet? Feels like we’ve gone backwards from the AIM Direct Connect of old.
It’s too big for email, and likely too big for Dropbox or Gdrive unless you have a paid account with them.
That means you’re going to have to get slightly technical. Find a freeware SFTP program that can spawn a server on the host, and connect to it from the client to download the file.
file.pizza if this is a one off or rare occurrence. If you're doing this regularly, there are better options, provided the person at the "source" computer is competent. A significant question is whether or not these computers are on the same network. I would recommend running a HTTP server if you don't care about privacy, HTTPS if you do. There's no need to buy an SSL certificate, self-signed is more than adequate for this purpose.
It's more complicated to set up, but the advantage is that when you're done you can send the receiving party a link they can open in any web browser, no hassle.
Sync by resilio. I use it between my computers and nas and between my computer and my vps and my nas and my vps, I also use it between my work and personal phone.
I've never had an issue but then again I don't use it for much more than movies and TV. If sync thing supports qnap I might check it out but I've been using sync since it was launched (if not soon after).