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Yeah, I've had probalems with files not being added to the library which I solved in the same way as you by using their preferred naming convention.
I'm talking about videos that play fine on my PC in VLC, but don't play in the Wii web browser. I considered getting a Chromecast dongle to use instead, haven't got round to it yet though.
On the other hand, there's always the Amazon video service / Nutflox.
I use Plex so I can stream media to my TV from my PC, via the web browser in my Wii U. It works about 90% of the time, depending on the encoding of the video.