EmailMessage().add_attachment() doesn't work for htm... | python/cpython

Bug report

I am trying to send a 1.5 MB .html file using from email.message import EmailMessage through the add_attachment method. If the filename ends in ".html", the email will not be sent, nor any warning/exception is raised.

This is the code that I am using:

message["From"] = sender_email
message["To"] = ", ".join(receiver_email)
message.set_content(text)

context = ssl.create_default_context()

for i, fn in enumerate([filename, mapname]):
    print("Processing filename", fn)
    with open(fn, "rb") as attachment:
        content = attachment.read()
        ctype, encoding = mimetypes.guess_type(fn)
        maintype, subtype = ctype.split("/", 1)
        message.add_attachment(
            content,
            maintype=maintype,
            subtype=subtype,
            filename=f"{i}_{fn}",
        )

with smtplib.SMTP_SSL("smtp.gmail.com", port, context=context) as server:
    server.login(sender_email, password)
    server.send_message(message)
    print("Sent!")

You see that I have two filenames, one is filename and the other one is mapname. filename is a csv file and mapname is an html file. If I only use the filename, the mail is sent. If I add mapname, the mail is not sent. If I only use mapname, the email is not sent either.

If the filename is changed: f"{i}_{fn.split('.')[0]}", so that the extension is removed, the email is sent! This shows that the HTML is not corrupt, it's just because of the extension! Also, I tried sending a simple html such as this one:

msg_content = """
<html><body>
  Hello world
</body></html>"""
content = bytes(msg_content, "utf-8")
ctype, encoding = mimetypes.guess_type("second.html")
maintype, subtype = ctype.split("/", 1)
message.add_attachment(
    content,
    maintype=maintype,
    subtype=subtype,
    filename="second.html",
)

And it still does not work.

Your environment

I'm using a virtual environment with Python version Python 3.9.10. The architecture is arm64. pip freeze shows:

absl-py==1.0.0
alembic==1.7.6
anyio==3.5.0
appnope==0.1.2
argon2-cffi==21.3.0
argon2-cffi-bindings==21.2.0
asttokens==2.0.5
attrs==21.4.0
autopage==0.5.0
Babel==2.9.1
backcall==0.2.0
beautifulsoup4==4.10.0
beniget==0.4.1
black==22.1.0
bleach==4.1.0
bokeh==2.4.2
certifi==2021.10.8
cffi==1.15.0
charset-normalizer==2.0.12
chex==0.1.1
click==8.0.4
cliff==3.10.1
cmaes==0.8.2
cmd2==2.4.0
colorlog==6.6.0
cycler==0.11.0
Cython==0.29.28
debugpy==1.5.1
decorator==5.1.1
defusedxml==0.7.1
dm-tree==0.1.6
dnspython==2.2.1
entrypoints==0.4
executing==0.8.3
flatbuffers==2.0
flax==0.4.0
fonttools==4.30.0
gast==0.5.3
idna==3.3
ipykernel==6.9.1
ipython==8.1.1
ipython-genutils==0.2.0
jedi==0.18.1
Jinja2==3.1.1
joblib==1.1.0
json5==0.9.6
jsonschema==4.4.0
jupyter-client==7.1.2
jupyter-core==4.9.2
jupyter-server==1.16.0
jupyterlab==3.3.2
jupyterlab-pygments==0.1.2
jupyterlab-server==2.12.0
kiwisolver==1.3.2
lightgbm==3.3.2
Mako==1.2.0
MarkupSafe==2.1.0
matplotlib==3.5.1
matplotlib-inline==0.1.3
miceforest==5.3.0
mistune==0.8.4
msgpack==1.0.3
mypy-extensions==0.4.3
nbclassic==0.3.7
nbclient==0.5.13
nbconvert==6.4.5
nbformat==5.2.0
nest-asyncio==1.5.4
notebook==6.4.10
notebook-shim==0.1.0
numpy==1.22.3
opt-einsum==3.3.0
optax==0.1.1
optuna==2.10.0
packaging==21.3
pandas==1.3.5
pandocfilters==1.5.0
parso==0.8.3
pathspec==0.9.0
pbr==5.8.1
pexpect==4.8.0
pickleshare==0.7.5
Pillow==9.0.1
platformdirs==2.5.1
ply==3.11
prettytable==3.2.0
prometheus-client==0.13.1
prompt-toolkit==3.0.28
ptyprocess==0.7.0
pure-eval==0.2.2
pybind11==2.9.1
pycparser==2.21
Pygments==2.11.2
pymongo==4.0.1
pyparsing==3.0.7
pyperclip==1.8.2
pyrsistent==0.18.1
python-dateutil==2.8.2
pythran==0.11.0
pytz==2021.3
PyYAML==6.0
pyzmq==22.3.0
requests==2.27.1
scipy==1.9.0.dev0+1659.2656ccb
Send2Trash==1.8.0
six==1.16.0
sniffio==1.2.0
soupsieve==2.3.1
SQLAlchemy==1.4.32
stack-data==0.2.0
stevedore==3.5.0
terminado==0.13.3
testpath==0.6.0
threadpoolctl==3.1.0
tomli==2.0.1
toolz==0.11.2
tornado==6.1
tqdm==4.63.0
traitlets==5.1.1
typing_extensions==4.1.1
urllib3==1.26.9
wcwidth==0.2.5
webencodings==0.5.1
websocket-client==1.3.2

EmailMessage().add_attachment() doesn't work for html files

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