Add attachment file

[setmao]

Let we can add attachment file when build mail.

Steps to Test This PR

The following steps are also performed and verified.

Send The Rendered Emails with Attachment File

$python3 send_mail.py example/mail_config.json --attachment_file ~/Desktop/社群軌主題及場地.pdf
You are about to send the mails under "mails_to_sent". Do you want to continue? [y/N]: y      
Please enter your mail account: set840526@gmail.com
Please enter you mail password:
INFO:root:Email sent to example@gmail.com!

Expected Result

The receivers could get the email with attachment file, and the file could open/download correctly.

[tai271828]

The feature is awesome and it will be useful to the sponsorship team definitely. There are two items still needing your help:

1. This pull request is very likely to raise change conflict after this pull request lands Add suffix option #4 You may need to resolve the conflict later.
2. May you please elaborate how to test your pull request? You may refer to this pull request as an example Add suffix option #4 (see "Steps to Test This PR")

[tai271828]

The code looks good to me. We just need further information and action described in this comment #5 (comment)

[setmao]

Thx @tai271828
I update more information for how to test this PR in first comment.

[Lee-W]

Overall, it looks great! Thanks for contributing! I leave 2 comments for minor issues.

[setmao]

@Lee-W Thx for your comment
I adding type annotation to attachment_file and use if attchement: in latest two commit.

[Lee-W]

The code looks good to me. Thanks for the contribution! The github action looks a bit weird now. I'll reword some of your commits and handle it on my side