无法使用SMTPLIB打开附件

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

msg = MIMEMultipart()

msg['From'] = 'sender@example.com'
msg['To'] = 'receiver@example.com'
msg['Subject'] = 'Email with Attachment'

body = 'This is the email body.'
msg.attach(MIMEText(body, 'plain'))

filename = 'attachment.txt'
attachment = open(filename, 'rb')

part = MIMEBase('application', 'octet-stream')
part.set_payload((attachment).read())
encoders.encode_base64(part)
part.add_header('Content-Disposition', "attachment; filename= %s" % filename)

msg.attach(part)

server = smtplib.SMTP('smtp.example.com', 587)
server.starttls()
server.login('sender@example.com', 'password')
text = msg.as_string()
server.sendmail('sender@example.com', 'receiver@example.com', text)
server.quit()