[SOLVED] How to send attachment on s/mime email

I'm trying to send an encrypted mail, using s/mime protocol, but the mail client doesn't acknowledge the attachments, and doesn't decrypt them. I've checked some answers on stack overflow, but none of them worked for me. Please note that the encryption itself does work (see below)

Here is my code:

import os
import ssl
from email.mime.text import MIMEText
from email.mime.application import MIMEApplication
from email.mime.multipart import MIMEMultipart
from smtplib import SMTP
from typing import List, Optional, Union

from M2Crypto import BIO, SMIME, X509

port = 587


def send_email_by_smtp(sender: str, receiver: List[str], message: Union[bytes, str]):
    # getting the credentials fron evironemnt
    host = os.environ.get("SMTP_FQDN")
    user = os.environ.get("SMTP_USER_ID")
    password = os.environ.get("SMTP_PASSWORD")

    # setting up ssl context
    context = ssl.create_default_context()

    # creating an unsecure smtp connection
    with SMTP(host, port) as server:
        # securing using tls
        server.starttls(context=context)

        # authenticating with the server to prove our identity
        server.login(user=user, password=password)

        # sending a plain text email
        return server.sendmail(sender, receiver, message)


def send_mail(
    *,
    recipient: List[str],
    subject: str,
    body: str,
    attachments: Optional[List[str]] = None,
):
    if attachments is None:
        attachments = []

    message = MIMEMultipart()
    from_address = os.environ.get("FROM_ADDRESS")

    body_part = MIMEText(body, "html")
    message.attach(body_part)
    for attachment in attachments:
        with open(attachment, "rb") as fh:
            attachment_part = MIMEApplication(fh.read())
            filename = os.path.basename(fh.name)
            attachment_part.add_header(
                "Content-Disposition",
                "attachment",
                filename=filename,
                Content_Transfer_Encoding="base64",
                Content_ID=f"<{filename}>",
                Content_Type="text/csv",
                Content_Disposition=f"attachment; filename={filename}",
            )
        message.attach(attachment_part)

    # data = message.as_string()
    data = encrypt_message(
        from_address,
        recipient,
        subject,
        message.as_bytes(),
        from_key="certs/new/signer_key.pem",
        from_cert="certs/new/signer.pem",
        to_certs=["certs/new/recipient.pem"],
    )
    return send_email_by_smtp(from_address, recipient, data)


def encrypt_message(from_addr, to_addrs, subject, msg, from_key, from_cert=None, to_certs=None):
    msg_bio = BIO.MemoryBuffer(msg)
    sign = from_key
    encrypt = to_certs
    p7 = None

    s = SMIME.SMIME()
    if sign:
        s.load_key(from_key, from_cert)
        if encrypt:
            p7 = s.sign(msg_bio, flags=SMIME.PKCS7_TEXT)
        else:
            p7 = s.sign(msg_bio, flags=SMIME.PKCS7_TEXT | SMIME.PKCS7_DETACHED)
        msg_bio = BIO.MemoryBuffer(msg)  # Recreate coz sign() has consumed it.

    if encrypt:
        sk = X509.X509_Stack()
        for x in to_certs:
            sk.push(X509.load_cert(x))
        s.set_x509_stack(sk)
        s.set_cipher(SMIME.Cipher("aes_256_cbc"))
        tmp_bio = BIO.MemoryBuffer()
        if sign:
            s.write(tmp_bio, p7)
        else:
            tmp_bio.write(msg)
        p7 = s.encrypt(tmp_bio)

    out = BIO.MemoryBuffer()
    out.write(f"From: {from_addr}\r\n")
    out.write(f"To: {', '.join(to_addrs)}\r\n")
    out.write(f"Subject: {subject}\r\n")
    if encrypt:
        s.write(out, p7)
    else:
        if sign:
            s.write(out, p7, msg_bio, SMIME.PKCS7_TEXT)
        else:
            out.write("\r\n")
            out.write(msg)
    out.close()

    return out.read()

if __name__ == "__main__":
    res = send_mail(recipient=os.environ.get("TO_ADDRESS", "").split(","), subject="some subject", body="some body", attachments=[__file__])
    print(res)

and the results on the client :

Content-Type: multipart/mixed; boundary="===============2300897735206349356=="
MIME-Version: 1.0

--===============2300897735206349356==
Content-Type: text/html; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

some body
--===============2300897735206349356==
Content-Type: application/octet-stream
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smtp_test.py";
 Content-Transfer-Encoding="base64"; Content-ID="<smtp_test.py>";
 Content-Type="text/csv"; Content-Disposition="attachment;
 filename=smtp_test.py"

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--===============2300897735206349356==--