I'm on Server Version 188.8.131.52 on Windows 10.
I don't know when this changed, but previously (I believe) Emby Server would recognize a VIDEO_TS folder in the Extras folder in the following directory format:
Dark City (1998)\Extras\Gallery and Metropolis Comparison (Text)\VIDEO_TS
And then Emby Theater would present that item "Gallery and Metropolis Comparison (Text)" in the lists of Extras, where it could be played via the External Player I have configured (in my case, VLC).
The reason I do this is that sometimes there are extras such as image galleries and text content which are only available via the DVD menu. So I copy only the files within the original VIDEO_TS folder which are necessary for the DVD-menu-only extras I want and put those in a folder like the above example.
Currently these folders are no longer being recognized and displayed for Movies, though it does still work for folders with the TV "Special" season folder. It is also possible that I am not remembering correctly and that it actually never worked for Movie the Extras folder and has only ever worked for TV Special folder. I don't think that is the case though -- I think it worked for the Movie Extras folder at some point in the past, as I normally test these things when I am converting a movie.
I understand this is an edge case which might not effect many users, but figured it wouldn't hurt to mention it in case it is something that can be easily fixed.
If folks have suggestions for how to achieve my above goals using a different route I'm open to that.