Show thumbnails of AVI and WMV files in finder
Apparently I was a bit trigger-happy with the bounty, because after a bit more Googling, I stumbled upon QLVideo, which:


  "-- allows macOS Finder to display thumbnails, static QuickLook previews, cover art and metadata for most types of video files."


The Github page has an installer (.pkg) under the releases page, but if you have Homebrew installed, you can also use brew cask install qlvideo on command line (that's what I used — and thumbnails started generating immediately). 

It even has couple of settings available:


  
  defaults write uk.org.marginal.qlvideo SnapshotAlways -bool XX
  
  
  Controls whether QuickLook "Preview" shows static snapshot(s) even if
  a playable preview is available. Default is NO.
  
  defaults write uk.org.marginal.qlvideo SnapshotCount -int XX
  
  
  Maximum number of
  snapshots to show in a QuickLook "Preview". Default is 10.
  
  defaults write uk.org.marginal.qlvideo SnapshotTime -int XX
  
  
  Time offset in
  seconds for thumbnails and for single snapshots. Unless the video clip
  is shorter than twice this value, in which case the snapshot is taken
  at the mid-point. Default is 60.
  This setting doesn't affect thumbnails that have already been generated - use qlmanage -r cache and re-start Finder to force regeneration of existing thumbnails.