i would try
clicking organize /folders and search options/view tab then make sure the box to show icons not thumbnails is unchecked. it depends on the filetype,you may need a third party software to display mkv and avi and more.
if the issue is the filetype then you could install icaros |MG| Icaros 2.0.0 Download
if you install icaros then when you open the software you can add this list to it to ensure these filetypes display their thumbs...
.mkv, .0gm, ,3g2, .3gp, .3gpp, ,avi, ,divç evo, .f4v, fly, hdmov,
.k3g, .m2p, ,mp2, ,m2v, .m4v, mod, ,mov, mpg, mpeg. ,m4p,
.mp4, .mp4v, .ogv, .qt, ,rm, .rmm, rrrìvb, .tp, .m2ts, .ts, ,mts, .vob,