"error": "Internal Server Error",
  "exception": "org.springframework.web.multipart.MultipartException",
  "message": "Current request is not a multipart request", "path": "/upload"

錯誤原因：Headers填寫錯誤，估計是之前選錯了x-www-form-urlencoded

由於上傳檔案用的Content-Type應該是multipart/form-data，修改之後依然報錯。

  "error": "Internal Server Error",
  "exception": "org.springframework.web.multipart.MultipartException",
  "message": "Could not parse multipart servlet request; nested exception is java.io.IOException: org.apache.tomcat.util.http.fileupload.FileUploadException: the request was rejected because no multipart boundary was found",
  "path": "/upload"

解析之後發現Content-Type實際用的是text/plain。但是這麼輸入依然報相同錯誤。

多次試驗之後發現，Content-Type無論選擇什麼，都會報錯。正確的做法是刪除Content-Type就行了。