upload multi file with golang http client

upload.go

package main

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"mime/multipart"
	"net/http"
	"os"
	"time"
)

func newFileRequest(uri string, fields map[string]interface{}, files map[string][]string) (*http.Request, error) {
	body := new(bytes.Buffer)
	writer := multipart.NewWriter(body)
	for key, val := range files {
		for i := range val {
			fi, err := os.Open(val[i])
			if err != nil {
				return nil, err
			}
			defer fi.Close()
			part, err := writer.CreateFormFile(key, fi.Name())
			if err != nil {
				return nil, err
			}
			if _, err := io.Copy(part, fi); err != nil {
				return nil, err
			}
		}
	}
	for key, val := range fields {
		writer.WriteField(key, fmt.Sprintf("%v", val))
	}
	if err := writer.Close(); err != nil {
		return nil, err
	}
	req, err := http.NewRequest("POST", uri, body)
	if err != nil {
		return nil, err
	}
	req.Header.Add("Content-Type", writer.FormDataContentType())
	return req, nil
}

func main() {
	fields := map[string]interface{}{
		"hostname":  "test_host",
		"timestamp": time.Now().Unix(),
	}
	files := map[string][]string{
		"file": []string{
			"./test.txt",
		},
	}
	request, err := newFileRequest("http://127.0.0.1:8000/upload", fields, files)
	if err != nil {
		panic(err)
	}
	client := &http.Client{}
	resp, err := client.Do(request)
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	b, _ := ioutil.ReadAll(resp.Body)
	fmt.Println(string(b))
}