What is the proper approach for managing requests containing both JSON and Multipart File in Spring Boot?

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The proper approach for managing requests containing both JSON and Multipart files in Spring Boot is to use the @RequestPart annotation to map the multipart files to a MultipartFile object, and the @RequestBody annotation to map the JSON payload to an object.

Here's an example code snippet:

@PostMapping("/upload")
public void upload(@RequestPart("file") MultipartFile file, 
                   @RequestBody User user) {
    // Handle the file and user object as needed
}

In this example, the MultipartFile object is mapped to the "file" part of the multipart request, and the User object is mapped to the JSON payload.

Note that when using multipart requests, the Content-Type header must be set to "multipart/form-data".