Ok found the problem the send of the flv or mp4 is being triggered by an X-Accel-Redirect header who's content type is text/html. It seems this header is maintained by nginx when processing the flv send instead of setting up one from the configured mime.types.

This is because the flv modules calls ngx_http_set_content_type to set this up which returns NGX_OK without making any changes if there is already content_type for the request. An existing content_type already seems to be being inherited from the original response containing the X-Accel-Redirect header, hence no change and the problem.

The docs clear state this is the case here: http://wiki.nginx.org/XSendfile

This doesn't effect mp4's which I was testing with originally as the mp4 module blindly sets video/mp4 so works fine in this case.

So sorry for the noise all a case of gotcha I'm afraid.