mail-parser treats multiparts as attachments

[ItayBenAvi]

Describe the bug
mail-parser fails on mails that have multipart attachments
mail-parsers fails in core.py:473
Traceback:
AttributeError: 'NoneType' object has no attribute 'decode'

This happens because of recent change in line core.py:353:
"or ported_string(p.get_content_disposition()).lower() == "attachment""

To Reproduce
Create a part which is multipart with content-disposition of attachment

Expected behavior
Payload should be treated as multipart and as attachment

It can be resolved by changing core.py:353 from 'or' to 'and' statement

Raw mail
Un-fortunately, I can't add the mail for security reason.
The mail have a part with the following payload:

• content-disposition: attachment
• p.is_multipart == True

Environment:

• OS: Linux
• Docker: yes
• mail-parser version 4.1.2 (doesn't re-produce on 3.5.0)