How to Prepare a multipart body of a response in Sprint Boot
use PartEvent for Reactive Stack To stream multipart data sequentially, you can provide multipart content through PartEvent objects. Form fields can be created via FormPartEvent::create. File uploads can be created via FilePartEvent::create. You can concatenate the streams returned from methods via Flux::concat, and create a request for the WebClient. For instance, this sample will POST a multipart form containing a form field and a file. return Flux.concat( FormPartEvent.creat