Currently, if your dvd extra, blooper reel, interview that isn't listed on thetvdb, you have few good options.
1. Add it to thetvdb,,, and pray that it isn't removed.
If you can't do option 1 because the show is locked, and no edits are allowed,,,,
2. Edit every single file name to S00Exx (where xx is a number that doesn't correspond with an existing entry on thetvdb)
What I currently do,,,
3. Create another category, a Home Videos category (mine is called MISC) and put all of these videos in this folder organized with subfolders. Then, from your plex client, view the category by folder (since the default view will sort by file name, and does not look pretty).
The Solution I propose:
In recent Plex updates, we can now use Extras folders for movies, and keep our extra content there. I propose that there be a subfolder name that Plex will recognize as something to not match with thetvdb, and serve as an Extras folder.
/TV Shows
/Doctor Who (2005)
/Season 00
Doctor Who (2005) - S00E02 - The Christmas Invasion.mkv
/Extras
Foxes performs 'Don't Stop Me Now' Doctor Who Series 8 (2014).mp4
/Season 3
Deleted Scenes.mkv
Please consider this in your efforts towards making the best Home Theatre software ever.