This problem is reported offline by Shane Kumpf.
When aws-sdk-go is used to access to s3 gateway of Ozone it sends the Multi Part Upload initialize message with "application/octet-stream" Content-Type.
This Content-Type is missing from the aws-cli which is used to reimplement s3 endpoint.
The problem is that we use the same rest endpoint for initialize and complete Multipart Upload request. For the completion we need the CompleteMultipartUploadRequest parameter which is parsed from the body.
For initialize we have an empty body which can't be serialized to CompleteMultipartUploadRequest.
The workaround is to set a specific content type from a filter which help up to create two different REST method for initialize and completion message.
Here is an example to test (using bogus AWS credentials).
Without the patch it returns with HTTP 405 (Not supported Media Type).