that thumbs.jpg is WMP's attempt at taking your cover artwork and shrinking it down so it can reverse it's mind and re-enlarge it and make it horribly distorted, (WMP should die a very slow and painful death). ....chances are if you change settings for Windows Explorer or whatever it's called in 8.1 you won't even see them or I would search .thumbs and just delete them all and the same with the .ini files....Google those file types so you understand what they are....then my next step would be to turn off WMP and send it deep into the bowls of your OP sys never to be seen again....there are tweaks to stop your OP sys from even making the .ini files as they don't speed anything up as far as I'm concerned. That's why they call them PERSONAL COMPUTERS! YOU ARE THE GOD....just saying.....lol
as for dragging and dropping your files.... why don't you import them into MC and associate all music type files to MC and be done with it? this way your assured not to have .thumbs or .ini files.....
you say playback stops when getting to the .thumbs or .ini labled tracks this confuses me as they are not "tracks"