@RequestPart cannot mapping string value，Required request part 'hostname' is not present

ServerCode
```
@PostMapping("/uploadLog")
public String uploadAnalysisLog(@RequestPart String hostname, @RequestPart MultipartFile runLog)
```

ClientCode
```
MultiValueMap form = new LinkedMultiValueMap<>();
form.add("runLog", fileResource);
form.add("hostname", hostname);
HttpEntity> body = new HttpEntity<>(form, headers);
RestTemplate restTemplate = HttpRestClient.getRestTemplateCse();
String result = restTemplate.postForObject(UPLOAD_LOG_URL, body, String.class);
```

Reproduce the multipart request using the uploadAnalysisLog endpoint and the RestTemplate.postForObject client shown in the issue. Trace how the @RequestPart hostname value is handled alongside runLog, then verify that the request is accepted without the required-part error.

索引モデルが issue の本文から書いたものです。