S3 output plugin not supporting "format" option

I'm having troubling getting Logstash to write to S3, and get the following error while grasping for straws:

`{:timestamp=>"2015-11-16T23:34:17.355000+0000", :message=>"Unknown setting 'format' for s3", :level=>:error}`

I'm using the Debian Logstash package:

```
# /opt/logstash/bin/logstash --version
logstash 2.0.0
```

Using an IAM role (where `aws s3 ls s3://...` works fine on the host server, given I install the awscli package from apt).

My `/etc/logstash/logstash.conf` is dead-simple:

```
input {
beats {
port => 5044
}
}

output {
file { path => "/var/log/logstash/messages.log" }
s3 {
bucket => "redacted"
prefix => "logstash/"
format => "plain"
}
}
```

Logstash is receiving JSON messages from the filebeat input just file. And the "file" output is working great as well. I'm at a loss as to how I can debug this further.