Why do files in Russian attached to a letter arrive broken?

Question

Python

In my application, some notifications are duplicated by sending a letter to the mail. Files uploaded by other users are attached to letters and they are often named in Russian. Empirically, it was calculated that if the file name is written in Latin, then everything comes fine, and those that have Cyrillic in the name come in .dat format.
It is also interesting that in mailboxes in the browser, files with Russian characters in the name can be displayed normally, but in Outlook the same letter will contain broken files.
Here is the code for sending the email:

if self.send_mail and not self.mail_was_send:
            self.mail_was_send = True
            msg = MIMEMultipart()
            msg["Subject"] = self.short_text
            msg["From"] = EMAIL_HOST_USER
            msg["To"] = self.user.email
            msg.attach(MIMEText(self.text))

            # получаем список файлов действвия и добавляем их в сообщение
            for document in documents if documents else []:
                path = os.path.join(MEDIA_ROOT, document.file.path)
                attachment = MIMEApplication(open(path, "rb").read(), _subtype=document.file.name.split(".")[-1],
                                             name=document.filename)
                attachment.add_header('Content-Disposition', 'attachment', filename=os.path.basename(path))
                msg.attach(attachment)

            s = smtplib.SMTP_SSL('smtp.yandex.ru', 465)
            s.ehlo(EMAIL_HOST_USER)
            s.login(EMAIL_HOST_USER, EMAIL_HOST_PASSWORD)
            s.sendmail(EMAIL_HOST_USER, [self.user.email], msg.as_string())
            s.quit()

Answer 1

How can I specify the encoding for a file name? If I can have both English and Russian files.

I don’t remember where it was originally stolen from (most likely SO), but the working version is

// set appropriate headers for attachment or streamed file
// The theoretically correct syntax for use of UTF-8 in Content-Disposition is very weird: filename*=UTF-8''foo%c3%a4 (yes, that's an asterisk, and no quotes except an empty single quote in the middle)
// This example is the same as the one above, but adding the "filename" parameter for compatibility with user agents not implementing RFC 5987:
// Content-Disposition: attachment;
//         filename="EURO rates";
//         filename*=utf-8''%e2%82%ac%20rates
// Note: Those user agents that do not support the RFC 5987 encoding ignore “filename*” when it occurs after “filename”.
// http://tools.ietf.org/html/rfc6266#section-5
// 
header('Content-Disposition: attachment; filename="'.rawurlencode($file_name).'"; filename*=utf-8\'\''.rawurlencode($file_name));