server: use 'application/octet-stream' mimetype as fallback
server: use 'application/octet-stream' mimetype as fallback

Instead of a mimetype of "null"

Fixes: https://github.com/brave/brave-browser/issues/5489

joshterrill/webtorrent,webtorrent/webtorrent,feross/webtorrent,joshterrill/webtorrent,feross/webtorrent,webtorrent/webtorrent