If those files are already in the mp4 format, and their file names are already containing the title, this can be quite painless with software like e.g. mp3tag:
I am using Plex as a plugin on FreeNAS, not that this really matters in the scenatio.
I have downloaded a playlist from YouTube and have named them in the following way:
GTA V - S01EX - Prologue.mp4
Where X is the episode number and then ‘Prologue’ is the unique title for each video.
I have added all these videos to the folder and created a Playlist and Plex has seen the Season and added all of the downloaded playlist together. This is great until I go to find a video and their titles in Ple…