[kh99]

Maybe I'm missing something, but if you've put this in postbit_legacy it will only show up once per post. If you wanted it to be inserted once per attachment you'd need a loop (in plugin code).

I also am not sure about this line

<if condition="$attachment[attachmentextension] != 'flv,mp4'">

If you're trying to check for two possible extensions I think you'd need something like

<if condition="!in_array($attachment[attachmentextension], array('flv','mp4'))">