I was just looking at the new API to figure out how to rewrite my old add-on for Firefox Quantum, and came across yours. Uploading and then downloading the engine data doesn't look like the most elegant solution, but if this is really the only thing you can do now, what you gonna do ¯\_(ツ)_/¯

I'd consider checking whether the downloaded file is the same as the one you uploaded though, just to be sure that you are getting back the same thing and the file host is possibly not trying to do anything funny.