Icons of m4v files missing after El Capitan update
There is a utility out there that does about all of the cleaning operations on a Mac O/S system, it's called Onyx and can be found on MacUpdate or thru your favorite search engine.

Go to the automate tab, check everything and let it run. Reboot when done. That should clean things up (caches, permissions, etc.) and is likely to fix this issue. It has for me.

Failing that my only other suggestion is to apply the latest combo update for El Capitan. Combo updates often fix a multitude of strange issues.