Long time user here... back from the MB 1.0 days! Anyways I've had an issue with Local Trailers not playing correctly for the past year or so now. I finally narrowed it down today and determined the cause.
There's two basic ways that local trailers can be stored and named. First is to have the trailer in the same folder as the movie file itself, with an identical name plus the -trailer suffix. Alternatively you can have a /Trailers sub-folder within each movie folder, as listed here..
Alternatively, trailers can also be stored in a trailers sub-folder. Bluray and Dvd folder rip structures must use a trailers sub-folder.
/Home Alone (1990)
Traditionally any file in the /Trailers sub folder would be played as a local trailer. It could be named "Trailer.mkv", "Transformerstrailer.mkv" "1.mkv", etc. The name itself didn't matter much, and it definitely didn't have to have the exact same name as the movie file in the main folder, or the exact same name as the main folder itself.
However, this seems to have changed and is broken. After much testing I have determined that NO local trailers play unless they are named identical to the main movie folders name, with the addition of the -trailer suffix. It doesn't matter that the trailer is in the /trailers sub folder. This completely negates the reason for having the subfolder method to begin with. Can someone please work on fixing this and having Emby Server recognize any file in the trailers subfolder as a trailer once more?