I tried to import a local video outside the library, using the media extended plugin.
The format is [test](<file:///D:/test.mp4>).
The video format is h264 encoded mp4, and it can be opened in edit mode, but it’s on a local player, so there’s no way to view it in obsidian. I only selected a few seconds of the video for testing purposes, but it still won’t play, as shown below:
Looking quickly at the Media Extended features, I don’t see any mention of embedding local files that are outside of a vault, but it seems to work in reading mode for me using this syntax (macOS):

If you drag the .mp4 into your vault and make a local ![[link.mp4]], does it work? It could be an encoding issue with the .mp4.
Embedding web links and viewing in reading mode also seems fine:
Media Extended enables embed/link from the following media sources:
Direct Link: 
YouTube: 
Vimeo: 
bilibili: 
Really appreciate your help. It works well when I drag the video into my vault and there’s no problem with embedding web links. Still have no idea why it can’t support video outside the vault. Maybe there’s sth wrong with my laptop(win 11)? I’ll find another way to avoid that.
I don’t regularly use Windows, but isn’t the startup drive/partition usually C:? I wonder if it’s because the file is on the D: partition? I’d play around with the target location; it could be a permission issue(?), but not sure. Good luck!