如何使用Python在现有的eml文件中添加pdf附件？

import email
from email.mime.multipart import MIMEMultipart
from email.mime.application import MIMEApplication

with open('existing.eml', 'rb') as f:
    msg = email.message_from_bytes(f.read())

new_msg = MIMEMultipart()

new_msg['From'] = msg['From']
new_msg['To'] = msg['To']
new_msg['Subject'] = msg['Subject']
new_msg.attach(msg.get_payload())

with open('attachment.pdf', 'rb') as f:
    attachment = MIMEApplication(f.read(), _subtype='pdf')

attachment.add_header('Content-Disposition', 'attachment', filename='attachment.pdf')

new_msg.attach(attachment)

with open('new.eml', 'wb') as f:
    f.write(new_msg.as_bytes())