Support multiple storage mime-types on S3

I added support for storing logs in S3 not only as GZip but also as JSON or plain text. This was useful to me in a current infra installation I was working on with Fluentd where logs needed to be uploaded to S3 as plain JSON rather than GZip.

I left defaults as they were, meaning that if you don't explicitly ask for a different storage format GZip will be used.