Greetings.
I've been working on getting Plex to see my extras. I was having an issue with extras for the movie 300. I'm on Yosemite, running Plex Media Server 0.9.12.19. The issue I encountered is that with the 3 extras I imported placed in a Featurettes subfolder (/Movies/300 (2006)/Featurettes/), they were being imported as distinct movies (complete with appropriate metadata about each extra) and displayed right next to the main title.
I tried renaming them a few times, pored over the instructions for Local Media Assets, and searched the forums for advice. By chance, I tried renaming the subfolder from Featurettes to Scenes. This worked. The extras are now displayed in the movie "pre-play" screen. Is the "featurettes" category not working on OS X?
On a side note, I already had all the extras working under Kodi, but wanted to try to get them into Plex. My solution for using a single directory of movies with extras working in both Kodi and Plex is to create a hardlink to each extra, give it a descriptive name for Plex, and place it in the appropriate Extras_type subfolder.