### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

I'm implementing the second example, using the transferManager described here:

https://docs.aws.amazon.com/AmazonS3/latest/userguide/example_s3_Scenario_UploadStream_section.html

```
        Long sizeOnS3;
        var key = dataSource.getObjectKey();

        S3TransferManager transferManager = S3TransferManager.builder()
                .s3Client(s3AsyncClient)
                .build();

        BlockingInputStreamAsyncRequestBody body =
                AsyncRequestBody.forBlockingInputStream(null);

        Upload upload = transferManager.upload(builder -> builder
                .requestBody(body)
                .putObjectRequest(req -> req.bucket(bucketName).key(key))
                .build());

        body.writeInputStream(dataSource.getDataStream());

        try {
            upload.completionFuture().get();
            sizeOnS3 = s3Client.headObject(b -> b.bucket(bucketName).key(dataSource.getObjectKey())).contentLength();
        } catch (Exception e) {
            log.error("Error while uploading file to S3", e);
            throw new RuntimeException(e);
        } finally {
            IOUtils.closeQuietly(dataSource.getDataStream());
        }
```

with the S3AsyncClient configure as such:

```
    @Bean
    public S3AsyncClient amazonAsyncS3Client(final Region region,
                                        final AwsCredentialsProvider awsCredentialsProvider,
                                        @Value("${application.aws.s3.uri}") final String maybeS3Uri) throws
            URISyntaxException {

        var s3ClientBuilder = S3AsyncClient.crtBuilder()
                .forcePathStyle(true)
                .region(region)
                .credentialsProvider(awsCredentialsProvider);

        if (StringUtils.isNotBlank(maybeS3Uri)) {
            s3ClientBuilder.endpointOverride(new URI(maybeS3Uri));
        }

        return s3ClientBuilder.build();
    }
```

The upload is successful but some extra data seems to be added before and after the original content:

original content (csv fetched from an API):

`MyData`

Content that I get if I fetch it again:

```
503
MyData

0
x-amz-checksum-crc32:e4f9zQ==
```

Deploying on AWS this extra content doesn't appear so it is my assumption that it is coming from localstack

### Expected Behavior

No extra information is added to the S3 file content other than the original data

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

docker-compose

  localstack:
    image: localstack/localstack:3.0.2
    ports:
      - "4566:4566" # Default port forward
    healthcheck:
      test: [ "CMD", "awslocal", "s3api", "wait", "bucket-exists", "--bucket", "stored-files" ]
      interval: 10s
      timeout: 5s
      retries: 20
    environment:
      - SERVICES=s3,sts
      - DEBUG=1
      - LAMBDA_REMOTE_DOCKER=0
      - DATA_DIR=/tmp/localstack/data
      - DEFAULT_REGION=eu-west-1
    volumes:
      - "${LOCALSTACK_VOLUME_DIR:-./volume}:/var/lib/localstack"
      - '/var/run/docker.sock:/var/run/docker.sock'
      - "./data/localstack/init.sh:/etc/localstack/init/ready.d/init.sh"


### Environment

```markdown
- OS:macOs 14.1.2
- LocalStack: 3.0.2
```


### Anything else?

_No response_
self,
        context: RequestContext,
        bucket: BucketName,
        key: ObjectKey,
        request_payer: RequestPayer = None,
        expected_bucket_owner: AccountId = None,
    ) -> GetObjectTorrentOutput:
        raise NotImplementedError
    def post_object(
        self, context: RequestContext, bucket: BucketName, body: IO[Body] = None
    ) -> PostResponse:
        
        
        
        
        
        store, s3_bucket = self._get_cross_account_bucket(context, bucket)
        form = context.request.form
        validate_post_policy(form)
        fileobj = context.request.files["file"]
        object_key = context.request.form.get("key")
        if "${filename}" in object_key:
            object_key = object_key.replace("${filename}", fileobj.filename)
        if canned_acl := form.get("acl"):
            validate_canned_acl(canned_acl)
            acp = get_canned_acl(canned_acl, owner=s3_bucket.owner)
        else:
            acp = get_canned_acl(BucketCannedACL.private, owner=s3_bucket.owner)
        post_system_settable_headers = [