Summary
At the moment, it is only possible to use root of the bucket. It will be nice to be able to use subfolder of the bucket. Some of us have more than 50 buckets...adding even single one more is chaos.

Expected behavior
Be able to set S3Prefix in order to use s3://bucketname/s3prefix/s3_object

Github: https://github.com/mattermost/mattermost-server/issues/14498