displaying flv-video attachment in post

The .flv is not a video player in itself, rather just a video file to be played in one.

To display it, you’ll need to play it from an actual video player. Unlike .wmv and .mp4, .flv files won’t play natively in your browser without one.

There’s a couple great open source options out there.

There are also a few plugins that might already have the functionality you’re looking for.