Почему при отправке письма с вложением на rambler приходит непонятный файл bin?

import os
import sys
import smtplib
import fnmatch
import codecs
from email.mime.multipart import MIMEMultipart
from email.mime.base import MIMEBase
from email.mime.text import MIMEText
from email import encoders
from email.utils import formataddr

def send_mail(smtp_addr,
              fromaddr,
              password,
              toaddr,
              sender=None,
              subject=None,
              text=None,
              data=None,
              ):
    from_addr = formataddr((sender, fromaddr))
    msg = MIMEMultipart()
    msg['From'] = from_addr
    msg['To'] = toaddr
    msg['Subject'] = subject or ''
    if text:
        msg.attach(MIMEText(*text))
    if data:
        for filename, body in data.items():
            attachment = MIMEBase('application', "octet-stream")
            attachment.set_payload(body)
            encoders.encode_base64(attachment)
            attachment.add_header('Content-Disposition',
                                  'attachment',
                                  # filename=quote(filename))
                                  filename=('windows-1251', '', filename))
            msg.attach(attachment)
    server = smtplib.SMTP_SSL('smtp.mail.ru', 465)
    server.login(fromaddr, password)
    text = msg.as_string()
    try:
        server.send_message(msg)
    except (smtplib.SMTPRecipientsRefused, smtplib.SMTPSenderRefused) as err:
        sys.stderr.write("Проблема с отправкой письма. Причина: %s" % err)
    finally:
        server.quit()

if __name__ == "__main__":
    mail_adr = 'mailfrom@mail.ru'
    mail_pass = 'password'
    m_to = 'mailto@rambler.ru'
    smtp_addr = 'smtp.mail.ru', 465
    fromaddr = mail_adr
    password = mail_pass
    toaddr = m_to
    subject = "Заголовок"
    sender = "Автор"
    text = 'Здравствуйте, это тест писма'
    data = {}
    path = r"d:\python\send_mail"
    fil = 'file.xls'
    matches = []
    for root, dirnames, filenames in os.walk(path):
        for filename in fnmatch.filter(filenames, fil):
            matches.append(os.path.join(root, filename))
    print(fil)
    with open(matches[0], "rb") as f:
        data[filename] = f.read()

    send_mail(smtp_addr,
              fromaddr,
              password,
              toaddr,
              sender=sender,
              subject=subject,
              text=(text, 'plain', '1251'),
              data=data)

    print('send mail ok')