How to realize the Upload asset with Java?

The first step was success, but when I do the second step, some weird exception occur. See the figure blow.

And I use Java webflux. If I didn’t specify the contentType, the exception is org.springframework.web.client.HttpServerErrorException: 412 <?xml version="1.0" encoding="UTF-8"?> <Error><Code>PreconditionFailed</Code><Message>At least one of the pre-conditions you specified did not hold</Message><Condition>Bucket POST must be of the enclosure-type multipart/form-data</Condition><RequestId>F7065D78A2180EB7</RequestId><HostId>3yDZCQajc6CpIrwqRAgfEQLjvPYVClXOFXKEajjX+jne+PMJkxvl3Oty0gOUs5t5Q/t2l5kkEsw=</HostId></Error>
Is there anyone have realize it in Java? Share with me, thank you very much.