Yeah, I will have to change that. It is like that, because to insert the "duration tag", we need to decode the entire video first and, for long videos, that's obviously going to take a while (or crash).

I recommend that you open the video in preview, and save it from there (with right click -> save video as).

EDIT: this should be "fixed". It will give you an error message, but you will be able to download the file, although it will not have the duration metadata.