best I can think of is internet search for the exact filename in quotes. AFAIK the hash is based on properties like torrent name which would he hard to guess maybe?
Piracy
Welcome to /c/piracy
No netflix or streaming services landlubbers allowed, this is pirates territory.
So, you have content of the torrent, but don't know the original torrent it goes to, right?
The first place I'd look would be bitsearch.to. It's a pretty extensive dht network crawler that actually has a lot of rarbg archived. Be warned, you can't use .'s or a few other 'special' characters, just replace them with spaces. If that fails to find anything, theres also btdig, but at this point you will probably want to install a program called jackett to search all public sites at the same time. Hopefully that would be a good tool for you.
When you find the torrent the file is from, you may need to find additional files, like advertisement txt files for dead torrent trackers. You may have some luck finding these in other torrents, you just have to match the filesize really.
If there aren't any seeders whatsoever, magnet links won't work. You will have to try and find the .torrent manually. You can pretty often find these on caching sites. The three I know of are itorrents.org, torrage.info, and btcache.me. Hopefully one of these still has the file.
When you go to add the torrent, set the stop condition in qbittorrent to files checked. This checks the integrity of the files you have on disk against the torrent you opened. Just be sure to save it to the same folder. If it checks without a hitch, you should be able to start seeding. Remember to port forward!
(Linux distro .mkv)
What?
EDIT: Oh wait I get it now. Sorry about that.