Golang AWS SES xlsx attachment corrupted

Question

In golang, I am trying to send an email with an XLSX file attached. I use github.com/tealeg/xlsx/v3 to generate the XLSX file as a byte array and it's working well when served with a web server (here gin) like that:

c.Header("Content-Description", "File Transfer")
c.Header("Content-Type", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")
c.Header("Content-Disposition", "attachment; filename="+time.Now().UTC().Format("daily-alerts-20060102.xlsx"))
c.Data(http.StatusOK, "application/octet-stream", fileContent)

But, when sent with SES, whereas a CSV file is served correctly, the 6ko XLSX file turns into a 10ko XLSX file, and it's impossible to open it with Excel:

func (s *EmailSenderParams) SendEmailWithAttachment(content string, data *models.RawEmailData, attachment []byte, attachmentFilename string) error {
    sess, err := session.NewSession(&aws.Config{
        Region: aws.String(s.awsRegion)},
    )

    creds := credentials.NewStaticCredentials(s.apiID, s.apiKey, "")

    // Create an SES session.
    svc := ses.New(sess, &aws.Config{Credentials: creds})

    // Assemble the email.
    buf := new(bytes.Buffer)
    writer := multipart.NewWriter(buf)

    // email main header:
    h := make(textproto.MIMEHeader)
    //h.Set("From", source)
    h.Set("To", data.ReceiverMail)
    //h.Set("Return-Path", source)
    h.Set("Subject", data.Subject)
    h.Set("Content-Language", "en-US")
    h.Set("Content-Type", "multipart/mixed; boundary=\""+writer.Boundary()+"\"")
    h.Set("MIME-Version", "1.0")
    _, err = writer.CreatePart(h)
    if err != nil {
        return err
    }

    // body:
    h = make(textproto.MIMEHeader)
    h.Set("Content-Transfer-Encoding", "7bit")
    h.Set("Content-Type", "text/plain; charset=us-ascii")
    part, err := writer.CreatePart(h)
    if err != nil {
        return err
    }
    _, err = part.Write([]byte(content))
    if err != nil {
        return err
    }

    // file attachment:
    h = make(textproto.MIMEHeader)
    h.Set("Content-Disposition", "attachment; filename="+attachmentFilename)
    h.Set("Content-Type", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet; x-unix-mode=0644; name=\""+attachmentFilename+"\"")
    h.Set("Content-Transfer-Encoding", "7bit")
    part, err = writer.CreatePart(h)
    if err != nil {
        return err
    }

    _, err = part.Write(attachment)
    if err != nil {
        return err
    }
    err = writer.Close()
    if err != nil {
        return err
    }

    // Strip boundary line before header (doesn't work with it present)
    st := buf.String()
    if strings.Count(st, "\n") < 2 {
        return fmt.Errorf("invalid e-mail content")
    }
    st = strings.SplitN(st, "\n", 2)[1]

    raw := ses.RawMessage{
        Data: []byte(st),
    }
    input := &ses.SendRawEmailInput{
        Destinations: []*string{aws.String(data.ReceiverMail)},
        Source:       aws.String(s.senderEmail),
        RawMessage:   &raw,
    }

    // Attempt to send the email.
    _, err = svc.SendRawEmail(input)

    // Display error messages if they occur.
    if err != nil {
        if aerr, ok := err.(awserr.Error); ok {
            switch aerr.Code() {
            case ses.ErrCodeMessageRejected:
                logrus.Warnln(ses.ErrCodeMessageRejected, aerr.Error())
            case ses.ErrCodeMailFromDomainNotVerifiedException:
                logrus.Warnln(ses.ErrCodeMailFromDomainNotVerifiedException, aerr.Error())
            case ses.ErrCodeConfigurationSetDoesNotExistException:
                logrus.Warnln(ses.ErrCodeConfigurationSetDoesNotExistException, aerr.Error())
            default:
                logrus.Warnln(aerr.Error())
            }
        } else {
            logrus.Warnln(err.Error())
        }

        logrus.Warnln(err)
        return err
    }

    logrus.Infoln("SES Email Sent to " + data.ReceiverName + " at address: " + data.ReceiverMail)

    return nil
}

I think there might be something wrong with MIMEHeaders, or with multipart or encoding, but I am strugling to find what. Do you know any successful method to send an email with an XLSX file attached with AWS SES?

Answer 1

The solution was to base64 encode the file, and set Content-Transfer-Encoding header to base64

    h.Set("Content-Transfer-Encoding", "base64")
    part, err = writer.CreatePart(h)
    if err != nil {
        return err
    }

    b := make([]byte, base64.StdEncoding.EncodedLen(len(attachment)))
    base64.StdEncoding.Encode(b, attachment)

    _, err = part.Write(b)