Golang - send an email with attachments.

sender.go

package main

import (
	"bytes"
	"encoding/base64"
	"fmt"
	"io/ioutil"
	"mime/multipart"
	"net/smtp"
	"os"
	"path/filepath"
)

var (
	host       = os.Getenv("EMAIL_HOST")
	username   = os.Getenv("EMAiL_USERNAME")
	password   = os.Getenv("EMAIL_PASSWORD")
	portNumber = os.Getenv("EMAIL_PORT")
)


type Sender struct {
	auth smtp.Auth
}

type Message struct {
	To          []string
	CC          []string
	BCC         []string
	Subject     string
	Body        string
	Attachments map[string][]byte
}

func New() *Sender {
	auth := smtp.PlainAuth("", username, password, host)
	return &Sender{auth}
}

func (s *Sender) Send(m *Message) error {
	return smtp.SendMail(fmt.Sprintf("%s:%s", host, portNumber), s.auth, username, m.To, m.ToBytes())
}

func NewMessage(s, b string) *Message {
	return &Message{Subject: s, Body: b, Attachments: make(map[string][]byte)}
}

func (m *Message) AttachFile(src string) error {
	b, err := ioutil.ReadFile(src)
	if err != nil {
		return err
	}

	_, fileName := filepath.Split(src)
	m.Attachments[fileName] = b
	return nil
}

func (m *Message) ToBytes() []byte {
	buf := bytes.NewBuffer(nil)
	withAttachments := len(m.Attachments) > 0
	buf.WriteString(fmt.Sprintf("Subject: %s\n", m.Subject))
	buf.WriteString(fmt.Sprintf("To: %s\n", strings.Join(m.To, ",")))
	if len(m.CC) > 0 {
		buf.WriteString(fmt.Sprintf("Cc: %s\n", strings.Join(m.CC, ",")))
	}
	
	if len(m.BCC) > 0 {
		buf.WriteString(fmt.Sprintf("Bcc: %s\n", strings.Join(m.BCC, ",")))
	}
	
	buf.WriteString("MIME-Version: 1.0\n")
	writer := multipart.NewWriter(buf)
	boundary := writer.Boundary()
	if withAttachments {
		buf.WriteString(fmt.Sprintf("Content-Type: multipart/mixed; boundary=%s\n", boundary))
		buf.WriteString(fmt.Sprintf("--%s\n", boundary))
	} else {
		buf.WriteString("Content-Type: text/plain; charset=utf-8\n")
	}

	buf.WriteString(m.Body)
	if withAttachments {
		for k, v := range m.Attachments {
			buf.WriteString(fmt.Sprintf("\n\n--%s\n", boundary))
			buf.WriteString(fmt.Sprintf("Content-Type: %s\n", http.DetectContentType(v)))
			buf.WriteString("Content-Transfer-Encoding: base64\n")
			buf.WriteString(fmt.Sprintf("Content-Disposition: attachment; filename=%s\n", k))

			b := make([]byte, base64.StdEncoding.EncodedLen(len(v)))
			base64.StdEncoding.Encode(b, v)
			buf.Write(b)
			buf.WriteString(fmt.Sprintf("\n--%s", boundary))
		}

		buf.WriteString("--")
	}

	return buf.Bytes()
}

func main() {
	sender := New()
	m := NewMessage("Test", "Body message.")
	m.To = []string{"[email protected]"}
	m.CC = []string{"[email protected]", "[email protected]"}
	m.BCC = []string{"[email protected]"}
	m.AttachFile("/path/to/file")
	fmt.Println(sender.Send(m))
}

Just wanna say thank you so much for this

If anybody is passing by, you can use

mime.TypeByExtension(filepath.Ext(filename))

instead of http.DetectContentType(byteSlice).

You'll likely have better results as the http function supports less MIME types.
Be aware, though, that the first function (from package mime) uses the file extension, which means that it can be tricked easily.
On the other hand, it seems that some file types have the same beginning bytes which will confuse the http function (.docx, .xls, etc apparently are mistaken for .zip, iirc).

Double check the last info but that should give you a good starting point to fix some of the issues people seemed to encounter in the comments above.