This is incorrect, we do *not* automatically use multipart upload for large files in the `put-object` command.  This is only true of the high level commands (in the `s3` namespace, not `s3api`).  This was changed in https://github.com/aws/aws-cli/commit/cbacc26c58b1d4c196f715727b6647dcd7a13cb1 as a response to https://github.com/aws/aws-cli/issues/752.

I'm going to go ahead and revert this change so that the multipart upload portion is removed.  Just want to give a heads up to make sure this makes its way back upstream.