Over the years, I collected a lot Youtube videos of either Music or Documantations or Tutorials of things, I deem important and so I like to collect those. I usually do this with Jdownloader.

But for example last night, I started downloading a lot of Timelapse videos and other nature or weather related things, which a lot of videos are 4k in quality.

We watch them in the living room on our Smart TV, all the videos are stored on our NAS. So tonight I wanted to show my gf some of the videos I downloaded, and our Smart TV wouldnt play those. I wondered, because we do that with a lot of downloaded Youtube videos, and the TV never complained.

I checked, so far the videos I have from youtube, were all .mp4. Now since last night and today, what I downloaded as all .mkv, which it seems our TV doesnt like so much, or lets say the TV's included Mediaplayer software.

Why is now out of a sudden all .mkv when it was til now always .mp4 ? Does it have to do with the 4k of the Youtube files ? Are 4k or larger files get converted by Jdownloader to .mkv or is that something that recently changed ?

How can I change that, or can I even ?
Thanks in advance.