类型错误：预期字符串有效负载：<class 'list'> 尝试发送带有 Python 3.9.6 中附加的 zip 文件的电子邮件时出错

Question

我使用以下代码（摘录）来发送电子邮件：

import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.application import MIMEApplication
from email.mime.base import MIMEBase
from email import encoders
smtpServer = "(Servername)"
smtpPort = (Portnumber)
smtpUsername = "(SMTP Username)" 
smtpPassword = "(Password)" 
sender = "(Username)" 
reciever = "(Username)"
subject = "(subject)"
body = "Text"
msg = MIMEMultipart()
msg['Subject'] = subject
msg['From'] = sender
msg['To'] = reciever
body = MIMEText(body, 'plain')
msg.attach(body)
msg = MIMEBase('application', 'octet-stream')
attach_file = open(zipfilename, 'rb')
payload = MIMEApplication(zipfilename)
payload.set_payload(attach_file.read(), 'rb')
encoders.encode_base64(payload)
payload.add_header('Content-Decomposition', 'attachment', filename=zipfilename)
msg.attach(payload)
smtpObj = smtplib.SMTP(smtpServer, smtpPort)
smtpObj.set_debuglevel(1)
smtpObj.starttls()
smtpObj.sendmail(sender, reciever, msg.as_string())
attach_file.close()
smtpObj.close()

我收到以下错误消息：

Traceback (most recent call last):
  File "E:\Data\testmail.py", line 31, in <module>
    smtpObj.sendmail(sender, reciever, msg.as_string())
  File "E:\Program\Python\3.9.6\lib\email\message.py", line 158, in as_string
    g.flatten(self, unixfrom=unixfrom)
  File "E:\Program\Python\3.9.6\lib\email\generator.py", line 116, in flatten
    self._write(msg)
  File "E:\Program\Python\3.9.6\lib\email\generator.py", line 181, in _write
    self._dispatch(msg)
  File "E:\Program\Python\3.9.6\lib\email\generator.py", line 218, in _dispatch
    meth(msg)
  File "E:\Program\Python\3.9.6\lib\email\generator.py", line 239, in _handle_text
    raise TypeError('string payload expected: %s' % type(payload))
TypeError: string payload expected: <class 'list'>

我已经在各种来源中进行了搜索（例如TypeError: string Payload Expected: <class 'bytes'>，https://github.com/Pylons/pyramid_mailer/issues/24，...），但我发现的提示不是'适合。失败的原因是什么？我能做些什么？