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I've downloaded a few videos from YouTube and added them to My usb stick but they won't play, they are mp4 1920x1080, I'll try to include a picture I took from my PlayStation.
I'll be honest and say I can't help as I've not used the media player as of yet. It might be worth asking on the official playstation forums to see if they know the answer. Others around here might be able to help you.