我正在嘗試通過 XMLHttpRequest 上傳文件，目前我正在使用 golang當(dāng)我使用 CURL 測試上傳時(shí)，一切正常，文件上傳到 Amazon S3 存儲桶，但在使用 Javascript 時(shí)出現(xiàn)以下錯(cuò)誤： multipart: NextPart: EOF這是 JS 部分：document.getElementById("attachment").addEventListener("change", function(e) {    var file = this.files[0];    var xhr = new XMLHttpRequest();    var formdata = new FormData();    formdata.append("attachment", file);    boundary=Math.random().toString().substr(2);    xhr.onreadystatechange = function(e) {        if ( 4 == this.readyState ) {            console.log(["xhr upload complete", e]);        }    };    xhr.open("POST", "https://upload_host:8443/upload", true);    xhr.setRequestHeader("Content-Type","multipart/form-data; charset=utf-8; boundary=---------------------------"+boundary+";");    xhr.send(formdata);}, false);對于 go 處理程序：func upload_handler(w http.ResponseWriter, r *http.Request, m render.Render) {  w.Header().Set("Access-Control-Allow-Origin", "*")  file, header, err := r.FormFile("C.Storage.FieldName")  if err != nil {    // ERROR THROWN HERE    ServeHTTP(400, err.Error(), m)    return  }  content, err := ioutil.ReadAll(file)  if err != nil {    ServeHTTP(400, err.Error(), m)    return  }  fileSize, err := file.Seek(0, 2) //2 = from end  if err != nil {    ServeHTTP(400, err.Error(), m)    return  }  if fileSize > int64(C.Storage.FileSize) {    ServeHTTP(400, "File size limit exceeded", m)    return  }  ftype := http.DetectContentType(content)  if !strings.Contains(C.Storage.AllowedMimes, ftype) {    ServeHTTP(400, "File type not allowed", m)    return  }  //FUNCTION TO UPLOAD TO AMAZON S3   file.Close()}