https://support.plex.tv/articles/local-files-for-trailers-and-extras/

Movies/MovieName (Release Date)/Extra_Directory_Type/Descriptive_name.ext
    Behind The Scenes
    Deleted Scenes
    Featurettes
    Interviews
    Scenes
    Shorts
    Trailers
    Other

Movies/MovieName (Release Date)/Descriptive_Name-Extra_Type.ext
    -behindthescenes
    -deleted
    -featurette
    -interview
    -scene
    -short
    -trailer
    -other