fail when attachment filename too long

[FeihuaLiu]

def send_mail(self):
    title = '【数据报表】' + self._title
    print(self._title)
    msg = Message(
        title,
        sender=('数据报表', 'admin@cheanjia.com'),
        recipients=list(current_app.config[
                            'CUSTOMIZED_QRCODE_SUBSCRIBE_DAILY_REPORT_RECIPIENTS']))

    xlsx = self.make_xlsx()
    msg.body = '''开始时间: %s\n结束时间: %s\n''' \
               '''数据见附件，如有问题请联系brad.liu@cheanjia.com''' % (
                   self.start_at.strftime('%Y-%m-%d %H:%M:%S'),
                   self.end_at.strftime('%Y-%m-%d %H:%M:%S'),
               )

    **# file_name='的打算打算大安大厦大大大'  fail
    # file_name = '的打算打算大安大厦大' pass
    # file_name = '的打算打算大安大厦大大' pass
    # file_name ='深圳测试中升雷克萨斯 - 二维码关注 - 20161216' fail**
    file_name = '二维码关注-%s' % (self.start_at.strftime('%Y%m%d'),)

    msg.attach(
        self._title + '.xlsx',  # 这个好像并没有什么用，什么字符串对收到的邮件都无影响
        'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
        xlsx.getvalue(),
        # 'attachment; filename="%s.xlsx"' % self._title
        'attachment; filename="%s.xlsx"' % file_name
        # 这个filename不能太长，否则会报错，发不出去
    )

    mail.send(msg)

I use Flask==0.10.1 and Flask-Mail==0.9.1 with python 3.5.1, to send email as above code . I found it will fail when attachment filename is long, as I comment fail. error log is below, I am not sure is bug of flask mail or python3.5.1, after google can't find how to fix this.

Traceback (most recent call last):
File "manage.py", line 31, in
manager.run()
File "/Users/brad/work/ZS-wecaht-web/venv/lib/python3.5/site-packages/flask_script/init.py", line 412, in run
result = self.handle(sys.argv[0], sys.argv[1:])
File "/Users/brad/work/ZS-wecaht-web/venv/lib/python3.5/site-packages/flask_script/init.py", line 383, in handle
res = handle(*args, **config)
File "/Users/brad/work/ZS-wecaht-web/venv/lib/python3.5/site-packages/flask_script/commands.py", line 216, in call
return self.run(*args, **kwargs)
File "/Users/brad/work/ZS-wecaht-web/app/commands/dev.py", line 281, in send_customized_qrcode_subscribe_daily_report
customized_qrcode_subscribe_daily_report(store_id)
File "/Users/brad/work/ZS-wecaht-web/app/reports.py", line 24, in customized_qrcode_subscribe_daily_report
report.send_mail()
File "/Users/brad/work/ZS-wecaht-web/app/reports.py", line 127, in send_mail
mail.send(msg)
File "/Users/brad/work/ZS-wecaht-web/venv/lib/python3.5/site-packages/flask_mail.py", line 492, in send
message.send(connection)
File "/Users/brad/work/ZS-wecaht-web/venv/lib/python3.5/site-packages/flask_mail.py", line 427, in send
connection.send(self)
File "/Users/brad/work/ZS-wecaht-web/venv/lib/python3.5/site-packages/flask_mail.py", line 190, in send
message.as_bytes() if PY3 else message.as_string(),
File "/Users/brad/work/ZS-wecaht-web/venv/lib/python3.5/site-packages/flask_mail.py", line 385, in as_bytes
return self._message().as_bytes()
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/message.py", line 179, in as_bytes
g.flatten(self, unixfrom=unixfrom)
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/generator.py", line 115, in flatten
self._write(msg)
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/generator.py", line 181, in _write
self._dispatch(msg)
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/generator.py", line 214, in _dispatch
meth(msg)
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/generator.py", line 272, in _handle_multipart
g.flatten(part, unixfrom=False, linesep=self._NL)
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/generator.py", line 115, in flatten
self._write(msg)
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/generator.py", line 195, in _write
self._write_headers(msg)
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/generator.py", line 422, in _write_headers
self._fp.write(self.policy.fold_binary(h, v))
File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/email/policy.py", line 192, in fold_binary
return folded.encode(charset, 'surrogateescape')
UnicodeEncodeError: 'ascii' codec can't encode characters in position 42-51: ordinal not in range(128)