Apparently there's no support yet for ISO / BDMV file type. I made a test with 10 files including MKV, AVI, M2TS, VOB and two ISO, with VTM and a few other tools (VCSI, MTN, Auto Movie Thumbnailer), none of them could create a preview for those two files. Among the media players, only VLC can read them correctly, but it doesn't have a thumbnail preview generation module (I tried SMPlayer, BSPlayer, PotPlayer, Media Player Classic).
As a workaround I'm gonna extract the .m2ts file with WinRAR, right now I only have those two so it won't be too much of a hassle, but native support would be nice.