That would explain why the original videos uploaded to the Unix server (not modified by the Unix system, ffmpeg or qt-faststart) and videos and previews on my localhost Windows system can be played by popcorn, at the same time all videos (including previews generated by the Unix-based extractor) can be played by Windows Media Player and Unix Movie Player and at the same time previews generated by a Unix-based video extractor cannot play in popcorn (it says it cannot decode them in the Firefox console).

Previews generated by the Unix-based extractor can play in Quicktime using the object and embed tags.

I don't think it's the video tag-else the original video would also not play in Popcorn, right?

Maybe we should replace popcorn due to incompatibility? QuickTime may need a browser-side plugin to play, but I believe it's more reliable. I already tested with Quicktime on the Unix test server accessible here

http://medici-dev.cyi.ac.cy:9000/

and it works.