Extra's on films

I have films on my Plex media server that have extra’s (Behind the scenes, Making of etc) how do I add them to the server, but associate them with a particular film. so they only come up when I select the film?

https://support.plex.tv/hc/en-us/articles/200220677-Local-Media-Assets-Movies

Look at the section for Local Extras.

You can place them in the same folder and follow the naming scheme or you can use sub-folders within the move folder.