GoLang: E-Mail mit Anhängen kann nicht gesendet werden

package mail

import (
    "bytes"
    logging "project/logging"
    "fmt"
    "log"
    "mime/multipart"
    "net/textproto"

    "gopkg.in/mail.v2"
)

func SendWithAttachment(from string, to []string, subject, bodyHTML string, attachmentName string, attachmentData []byte) error {
    logoURL := "/path/to/cs.jpg"
    m := mail.NewMessage()
    m.SetHeader("From", from)
    m.SetHeader("To", to...)
    m.SetHeader("Subject", subject)
    m.Embed(logoURL)

    // Create a multipart message
    body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)

    // Add HTML part
    htmlPart := make(textproto.MIMEHeader)
    htmlPart.Set("Content-Type", "text/html")
    htmlPart.Set("Content-Transfer-Encoding", "quoted-printable")
    htmlPartWriter, err := writer.CreatePart(htmlPart)
    if err != nil {
        logging.NewDefaultLogger().Errorf("Error creating html part: %s", err)
        return err
    }
    htmlPartWriter.Write([]byte(bodyHTML))

    // Add Attachment part
    attachmentPart := make(textproto.MIMEHeader)
    attachmentPart.Set("Content-Type", "application/octet-stream")
    attachmentPart.Set("Content-Transfer-Encoding", "base64")
    attachmentPart.Set("Content-Disposition", `attachment; filename="`+attachmentName+`"`)
    attachmentPartWriter, err := writer.CreatePart(attachmentPart)
    if err != nil {
        logging.NewDefaultLogger().Errorf("Error creating attachment part: %s", err)
        return err
    }
    attachmentPartWriter.Write(attachmentData)

    writer.Close()

    m.SetBody("multipart/mixed", body.String())

    fmt.Printf("Sending mail.....")
    err = gs.dialer.DialAndSend(m)
    if err != nil {
        logging.NewDefaultLogger().Errorf("Error sending email..: %s", err)
        return err
    }
    return nil
}

m := mail.NewMessage()
    m.SetHeader("From", emailDetails.From)
    m.SetHeader("To", emailDetails.To...)
    m.SetHeader("Subject", emailDetails.Subject)

    if emailDetails.BodyHTML != "" {
        m.SetBody("text/html", emailDetails.BodyHTML)
    } else {
        m.SetBody("text/plain", emailDetails.BodyPlain)
    }

    if emailDetails.AttachmentName != "" && len(emailDetails.AttachmentData) > 0 {
        m.Attach(emailDetails.AttachmentName, mail.SetCopyFunc(func(writer io.Writer) error {
            _, err := writer.Write(emailDetails.AttachmentData)
            return err
        }))
    }