如何发送电子邮件附件？

import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.base import MIMEBase
from email.mime.text import MIMEText
from email.utils import COMMASPACE
from email import encoders

# 邮件发送者和收件人
from_addr = 'sender@example.com'
to_addr = 'recipient@example.com'

# 创建邮件对象
msg = MIMEMultipart()
msg['From'] = from_addr
msg['To'] = to_addr
msg['Subject'] = '带附件的邮件'

# 添加邮件正文
body = '这是一个带附件的邮件。'
msg.attach(MIMEText(body, 'plain'))

# 添加附件
attachment = MIMEBase('application', 'octet-stream')
with open('example.txt', 'rb') as f:
    attachment.set_payload(f.read())
encoders.encode_base64(attachment)
attachment.add_header('Content-Disposition', f"attachment; filename={'example.txt'}")
msg.attach(attachment)

# 发送邮件
smtp_obj = smtplib.SMTP('smtp.example.com', 587)
smtp_obj.starttls()
smtp_obj.login('username', 'password')
smtp_obj.sendmail(from_addr, to_addr, msg.as_string())
smtp_obj.quit()

import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
import java.io.*;

public class SendEmailWithAttachment {
    public static void main(String[] args) {
        String to = "recipient@example.com";
        String from = "sender@example.com";
        String host = "smtp.example.com";
        Properties properties = System.getProperties();
        properties.setProperty("mail.smtp.host", host);
        properties.setProperty("mail.smtp.auth", "true");
        properties.setProperty("mail.smtp.starttls.enable", "true");
        properties.setProperty("mail.smtp.port", "587");
        Session session = Session.getInstance(properties, new Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication("username", "password");
            }
        });
        try {
            MimeMessage message = new MimeMessage(session);
            message.setFrom(new InternetAddress(from));
            message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
            message.setSubject("带附件的邮件");
            MimeBodyPart mimeBodyPart = new MimeBodyPart();
            mimeBodyPart.setText("这是一个带附件的邮件。");
            Multipart multipart = new MimeMultipart();
            multipart.addBodyPart(mimeBodyPart);
            mimeBodyPart = new MimeBodyPart();
            FileDataSource fileDataSource = new FileDataSource("example.txt") {
                @Override
                public String getContentType() {
                    return "application/octet-stream";
                }
            };
            mimeBodyPart.setDataHandler(new DataHandler(fileDataSource));
            mimeBodyPart.setFileName("example.txt");
            multipart.addBodyPart(mimeBodyPart);
            message.setContent(multipart);
            Transport.send(message);
        } catch (MessagingException e) {
            e.printStackTrace();
        }
    }
}