What are the large .f136 and .f171 files in my Downloads folder?

Today I discovered two large files in my Downloads folder:

$ ls -laht
total 19283784
-rw-r--r--@  1 mmorin  staff   6.0K  9 Sep 11:39 .DS_Store
drwx------+ 11 mmorin  staff   352B  9 Sep 11:39 .
drwxr-xr-x+ 65 mmorin  staff   2.0K  2 Sep 09:43 ..
-rw-r--r--   1 mmorin  staff   495M 25 Sep  2018 .f171
-rw-r--r--   1 mmorin  staff   8.7G 24 Sep  2018 .f136
-rw-------   1 mmorin  staff     0B 29 Jan  2018 .localized

They haven't been modified in one year. A cat of .f171 showed it was a binary file. I removed both files and the available space on the computer did not increase by 9.2 gigabytes immediately as I expected but it did after a day. I can recover them from a backup if needed.

What are these files?

I found the following comment on Reddit.

tl;dr you need to add ffmpeg or avconv executables to your PATH Sounds less like a problem with youtube-dl and more like you do not have ffmpeg or avconv on your PATH. Without one of the two, the streams cannot be muxed into a single file, so you're winding up with two files named .f136 and .f251 then you have a separate audio and video files, where you probably just want the single file with both audio and video streams contained in MP4/MKV/WEBM.

and,

They are separate audio and video files. You don't have ffmpeg installed and working, so it can't merge them into a single file. Install ffmpeg and make sure to add it to your path (google how to do this).

I cannot find anything about a .f171 file.