Forwarding a file upload

I'm working on an api endpoint in go that will accept an upload and then immediately forward to another API. I don't want to write the file to disk anywhere, but I'm not sure storing the file temporarily in memory the way I have is correct either. All the examples that I can find deal with saving the file to disk. I've posted what I'm doing below. The response I get back from the second API is that I failed to post a file, but I can see that it is receiving the "userID" field. Can someone please point out what I'm doing wrong as well as possibly advise if this is the best way to go about this?

Route Handler

func (r *Routes) forwardFile(w http.ResponseWriter, req *http.Request){ 
    parameters := mux.Vars(req)
    userID := parameters["userID"]

    const maxFileSize = 1 * 1024 * 1024 // 1MB

    // pull in the uploaded file into memory
    req.ParseMultipartForm(maxFileSize)


    file, fileHeader, err := req.FormFile("fileUpload")
	if err != nil {
		encodeResponse(w, req, response{obj: nil, err: err})
		return
	}
	defer file.Close()

	success, err := service.DoForwardFile(userID, file, fileHeader)
	encodeResponse(w, req, response{obj: success, err: err})
}

Service Handler

func (b *base) DoForwardFile(userID int, file multipart.File, fileHeader *multipart.FileHeader) (FileForwardedResponse, error) {
    // start building our request to forward the file
  	var resp *http.Response
  	defer func() {
  		if resp != nil {
  			resp.Body.Close()
  		}
  		reportStat.Complete(0)
  	}()
  
  	// build a form body
  	body := &bytes.Buffer{}
  	bodyWriter := multipart.NewWriter(body)
  
  	// add form fields
  	bodyWriter.WriteField("userID", userID)
  
  	// add a form file to the body
  	fileWriter, err := bodyWriter.CreateFormFile("fileUpload", fileHeader.Filename)
  	if err != nil {
  		return FileForwardedResponse{}, err
  	}
  	// copy the file into the fileWriter
  	_, err = io.Copy(fileWriter, file)
  	if err != nil {
  		return FileForwardedResponse{}, err
  	}
  	
  	// Close the body writer
  	bodyWriter.Close()
  
  	// build request url
  	apiURL := fmt.Sprintf("%s/v2/users/%d/files", config.APIURL, userID)
  	
  	// send request
  	client := &http.Client{Timeout: time.Second * 10}
  	req, err := http.NewRequest("POST", apiURL, body)
  	resp, err = client.Do(req)
  	
  	...	

  
  }

You're not setting the Content-Type for the request. Even if the header gets set automatically to multipart/form-data, it's missing the data boundary.

req, err := http.NewRequest("POST", uri, body)
if err != nil {
    return FileForwardedResponse{}, err
}
req.Header.Set("Content-Type", bodyWriter.FormDataContentType())
...