Cool. Code seems to work for me:

        location "/.well-known/erik/index/*" {
                header set "Content-Type" "application/rpki-erikindex"
		...
	}

$ curl -I -s --out-null -w '%header{content-type}\n' https://miso.sobornost.net/.well-known/erik/index/rpki.ripe.net
application/rpki-erikindex