/playsound command not playing sounds with custom file names

When creating a sound with a custom file name, and using the playsound command to play that sound, it does not play. For example if a folder called custom is created in a resourcepack, and a custom .ogg file with a custom name is placed in it, you would normally to play the sound in game use the command "/playsound custom.FILENAME PLAYERNAME", but no sound plays.
I don't know if this is a bug, or just a missing feature.