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