I can open the video with VLC and play it perfectly, but the fact that it is just "file" and not mp4 just really bothers me
I dont think that youtube can also run this type of file
Anyway to fix it?

For the current file: rename it such ash: filename.mp4

For the future: I think the extension is added automatically by KDENLive and derived from the rendering (= encoding) profile you choose from the list..