Header Content-Disposition incorrectly folded

[citizen-stig]

Here is a minimum working example:

import sys
from flask import Flask
from flask_mail import Message, Mail

app = Flask(__name__)
app.config['MAIL_SERVER'] = 'some.server.example.com'
app.config['MAIL_PORT'] = 587
app.config['MAIL_USE_TLS'] = True
app.config['MAIL_USERNAME'] = 'username'
app.config['MAIL_PASSWORD'] = 'password'
Mail(app)


@app.route('/')
def hello_world():
    filename = 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.txt'
    msg = Message('subj', recipients=['[email protected]'],
                  body='this is folding check', sender='[email protected]')
    msg.attach(filename, 'text/plain', b'hello world.\n\nby NG')
    msg_bytes = msg.as_bytes()
    sys.stdout.buffer.write(msg_bytes)
    response = ''.join(['<pre>' + x.strip() + '</pre>'
                        for x in msg_bytes.decode('utf-8').split('\n')])
    return response


if __name__ == '__main__':
    app.run()

And error part:

--===============8337733259214382862==
Content-Type: text/plain
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Disposition: attachment;filen
ame="AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.txt"
aGVsbG8gd29ybGQuCgpieSBORw==
--===============8337733259214382862==--

It inserts line separator in filename parameter, and some mail servers fail to proceed it.

It should be:

Content-Disposition: attachment;
 filename="AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.txt"
aGVsbG8gd29ybGQuCgpieSBORw==