How do I play .dvdmedia files in Mojave?

Remove the .dvdmedia extension on file. The file will now appear as a folder in Finder. Next, press ⌘Space, and launch the DVD Player app. In DVD Player, press ⌘O, and then open the VIDEO_TS folder in the DVD media folder that you just renamed.

For example, if you have a .dvdmedia file named GREAT_FILM.dvdmedia:

1. Rename GREAT_FILM.dvdmedia to GREAT_FILM.
2. Press ⌘Space to launch Spotlight.
3. In Spotlight, type "DVD Player," and then press Enter.
4. In DVD Player press ⌘O.
5. Navigate to the GREAT_FILM > VIDEO_TS folder, and then press Open.

Hope this helps!