Attach is not valid on a message with a non-multipart payload

Question

メールに書類を添付して送りたいです。

def mail(mail_adress, docx):
        smtp_host = 'smtp.gmail.com'
        smtp_port = 587
        username = 'zzz'
        password = 'zzzzz'


        msg = message.EmailMessage()
        msg.set_content('test mail')  
        msg['Subject'] = 'test mail(sub)'  
        msg['From'] = '[e-mail]'  
        msg['To'] = '[e-mail]'

        mine = {'type': 'text', 'subtype': 'docx'}
        attachment = MIMEBase(mine['type'], mine['subtype'])

        attach_file = {'name': 'test.docx', 'path': '/Users//test.docx'}
        file = open(attach_file['path'], 'rb')
        attachment.set_payload(file.read())
        file.close()
        encoders.encode_base64(attachment)
        msg.attach(attachment)
        attachment.add_header("Content-Disposition", "attachment", filename=attach_file['name'])

        server = smtplib.SMTP(smtp_host, smtp_port)
        server.ehlo()
        server.starttls()
        server.ehlo()
        server.login(username, password)
        server.send_message(msg)
        server.quit()

とコードを書きました。このコードを実行すると、
Attach is not valid on a message with a non-multipart payload　とエラーが出ました。Tracebackでは、

Traceback (most recent call last): File "/Users/xxx/anaconda3/envs/py36/lib/python3.6/site-packages/django/core/handlers/exception.py", line 35, in inner response = get_response(request) File "/Users/xxx/anaconda3/envs/py36/lib/python3.6/site-packages/django/core/handlers/base.py", line 128, in _get_response response = self.process_exception_by_middleware(e, request) File "/Users/xxx/anaconda3/envs/py36/lib/python3.6/site-packages/django/core/handlers/base.py", line 126, in _get_response response = wrapped_callback(request, *callback_args, **callback_kwargs) File "/Users/xxx/Downloads/legal_doc_mail/common/views.py", line 401, in make_document mail(mail_adress, file) File "/Users/xxx/Downloads/legal_doc_mail/common/views.py", line 472, in mail msg.attach(attachment) File "/Users/xxx/anaconda3/envs/py36/lib/python3.6/email/message.py", line 210, in attach raise TypeError("Attach is not valid on a message with a" TypeError: Attach is not valid on a message with a non-multipart payload

とエラーが出ました。
どのように修正すればいいでしょうか？何が原因でこのエラーが出るのでしょうか？