Had issues displaying thumbnails for MKV files that use H.265 encoding. Possibly H.264 MKV’s have same issue, but I was having this issue with MKV’s encoded with H.265. I only had generic media player icon. After digging around I managed to find a tool called “Icaros” made by a guy named Xanashi.
It’s pretty straight forward tool that solved my issue of thumbnails on MKV files. MKV files now display a thumbnail of its video content instead of just generic icon. Hurray 😀
Sidenote: Use 7-zip to unpack ZIP archive of the portable version. Windows is not able to extract it.