An example of something that does this is GPG, when operating in PGP/MIME
mode.
The Python documentation remarks that an attachment is anything that isn't
a body - meaning that if there are multipart messages nested within each
other, the containers will be flagged as an attachment.
When get_content() is called on the attachment, it fails with an unhandled
KeyError as there is no attachment handler for multipart.
This change wraps the get_content() call in a try...catch, and returns if
an attachment type unsupported by the runtime is present.
```
Traceback (most recent call last):
File "/Users/adamb/mail_to_misp/tests/tests.py", line 89, in test_nested_mime
self.mail2misp.process_email_body()
File "./mail2misp/mail2misp.py", line 188, in process_email_body
self._find_attached_forward()
File "./mail2misp/mail2misp.py", line 88, in _find_attached_forward
attachment_content = attachment.get_content()
File "/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/email/message.py", line 1096, in get_content
return content_manager.get_content(self, *args, **kw)
File "/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/email/contentmanager.py", line 25, in get_content
raise KeyError(content_type)
KeyError: 'multipart/mixed'
```