I have just updated both Mavic and original goggles to the new 1.4.200 firmware which should support replaying user videos according to the change log.
Has someone been successful with playing own videos with this new firmware firmware?
On the SD card there is a new folder USER_MEDIA to where I have copied various MP4 videos in 1k/30fps resolution.
The goggles will display very low-resolution thumbnail images out of the videos and when  trying to play a vvideo I will just get an empty DJI splash sceen with  timeline bar and hear the sound track of the video.
What's the trick to replay a video?   

I've played 1920x1080P @ 30 & 60 fps in .mp4, and so far played 1920x1080P in .mov without problems. Like you I dropped them into user media folder. Using a Sandisk Ultra 64GB micro sd card.

did you come right yet - i do not know the answer exactly but seem to remember something about having to go to the playback function on your devise and then download to the goggles - sorry its a vague answer but maybe you can work it out from there - i think i saw a you tube video on it - check out you tube