如何将msg中的附件附加到Mime中，以便在Python中作为电子邮件发送？

import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.image import MIMEImage
from email.mime.application import MIMEApplication

# 设置发件人、收件人和邮件主题
sender = "your_email@example.com"
receiver = "recipient_email@example.com"
subject = "Sending Email with Attachment"

# 创建一个MIMEMultipart对象作为邮件的根容器
msg = MIMEMultipart()
msg["From"] = sender
msg["To"] = receiver
msg["Subject"] = subject

# 添加正文内容
body = "This is the email body."
msg.attach(MIMEText(body, "plain"))

# 添加附件
attachment = "path_to_attachment.pdf"  # 附件的文件路径
with open(attachment, "rb") as file:
    attachment_part = MIMEApplication(file.read(), Name="attachment.pdf")
attachment_part["Content-Disposition"] = 'attachment; filename="attachment.pdf"'
msg.attach(attachment_part)

# 连接SMTP服务器并发送邮件
smtp_server = "smtp.example.com"
smtp_port = 587
username = "your_username"
password = "your_password"

try:
    with smtplib.SMTP(smtp_server, smtp_port) as server:
        server.ehlo()
        server.starttls()
        server.login(username, password)
        server.sendmail(sender, receiver, msg.as_string())
    print("Email sent successfully!")
except Exception as e:
    print("Failed to send email. Error:", str(e))