无法使用flask mail python向邮件添加附件

from flask import Flask
from flask_mail import Mail, Message

app = Flask(__name__)
app.config['MAIL_SERVER'] = 'smtp.qq.com'  # 邮件服务器
app.config['MAIL_PORT'] = 465  # 邮件服务器端口
app.config['MAIL_USE_SSL'] = True  # 启用SSL加密
app.config['MAIL_USERNAME'] = 'your-email@qq.com'  # 发送邮件的邮箱
app.config['MAIL_PASSWORD'] = 'your-email-password'  # 发送邮件的邮箱密码

mail = Mail(app)

msg = Message('邮件主题', sender='your-email@qq.com', recipients=['recipient@example.com'])

with app.open_resource('path/to/attachment.pdf') as attachment:
    msg.attach('attachment.pdf', 'application/pdf', attachment.read())

mail.send(msg)