Hi,
I hadn't noticed that bug before, so thanks for the feedback and letting me know of it! And I've tried dragging in files from folder other than Videos and it seems to work fine, though I'll still look into trying it from various other drives, including external ones, to see if I can replicate and try to resolve this issue.
And as for playing MKV files, the file format isn't supported natively by nodeJS or chromium (what Slate is built on top of) and I believe that's why Slate isn't able to play them. There are a few hacky ways that I've tried to get them playing, but that resulted in a bad experience overall including things like the video being played while the sound was absent. Though at the same time support for multiple media formats as well as support for certain streaming services are still on the roadmap and the aim is to be able to view as many media types as possible, so I'll try my best to fix the problems you've mentioned.
Thanks,
Ayan