I think the codecs on your computer need updating baker. In particular the H.264 codec which is what GoPro uses as it's recording format. Video shows as black screen on your computer, file size appears correct, the video you sent GoPro works for them - this leads me to a software issue on your computer.
Go to this site and download the K-Lite Mega Codec Pack -
http://www.free-codecs.com/download/K_Lite_Mega_Codec_Pack.htm
It includes the x264 decoder and should hopefully solve your problem. The pack also has many other codecs so that you don't run in to this problem with other video formats.
The download is safe. I have been using K-Lite for many years, it is one of the most popular codec packs available. I typically install it without any players, all you want is the decoders.