Object is shiny + thumbnail is dark

You can make your own thumbnail very easily. It will be a PNG file that you need to make. In fact, I recommend making a custom thumbnail if you plan to share this. It's easier to identify an item in the game if there's a custom thumbnail. Why the game didn't generate a thumbnail for you I can't help you with. 

As for your texture, the specular has a white alpha. Open the specular in Photoshop (or Gimp), delete the alpha channel, and save the texture as a DDS using DXT1 no alpha. Make sure generate mip maps has been selected. (Ignore the left side of this image. I'm sure your specular will already be flattened). 

You can use a PNG specular but in my opinion, it's tricky to make. I find using a DDS to be easier. In case you aren't aware of this, you will need a DDS plugin for Photoshop or Gimp.