Following this issue: https://bugs.python.org/issue33972

Same bug apply to email.message.get_body() with attached email example and the following code: 

        from email.policy import default
        import email

        with open('email_bad_formatted.eml', 'rb') as fp:
            msg = email.message_from_binary_file(fp, policy=default)
        body = msg.get_body()

> Result:
E       AttributeError: 'str' object has no attribute 'is_attachment'

/usr/local/lib/python3.9/email/message.py:978: AttributeError

Attached PR fix the issue.

Can you see how it happened that part is a str?

Yes, that's the real question.  That's what needs to be fixed, otherwise we'll just keep finding new bugs.  For example, try calling iter_parts() on that message.  It isn't pretty :)

Actually, I'm wrong.  The body of a part can be a string, and that's what's going to happen with a malformed body of something claiming to be a multipart. The problem is that there is code that doesn't guard against this possibility.  The following patch against master fixes the bug listed here, as well as iter_parts().  But it causes one test suite failure so it isn't a correct patch as it stands:

diff --git a/Lib/email/message.py b/Lib/email/message.py
index 3701b30553..d5d4a2385a 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -982,7 +982,7 @@ def _find_body(self, part, preferencelist):
             if subtype in preferencelist:
                 yield (preferencelist.index(subtype), part)
             return
-        if maintype != 'multipart':
+        if maintype != 'multipart' or not self.is_multipart():
             return
         if subtype != 'related':
             for subpart in part.iter_parts():
@@ -1087,7 +1087,7 @@ def iter_parts(self):
 
         Return an empty iterator for a non-multipart.
         """
-        if self.get_content_maintype() == 'multipart':
+        if self.is_multipart():
             yield from self.get_payload()
 
     def get_content(self, *args, content_manager=None, **kw):

Maybe someone can take this and finish it (with tests)...I may or may not have time to get back to this.

I'll try to fix the unit test today.

Put up the PR here: https://github.com/python/cpython/pull/26903

New changeset e3f877c32d7cccb734f45310f26beeec793364ce by andrei kulakov in branch 'main':
bpo-42892: fix email multipart attribute error (GH-26903)
https://github.com/python/cpython/commit/e3f877c32d7cccb734f45310f26beeec793364ce

New changeset 6f950023c6a2168b229363d75f59a24ecdd66d19 by Miss Islington (bot) in branch '3.10':
bpo-42892: fix email multipart attribute error (GH-26903) (GH-27492)
https://github.com/python/cpython/commit/6f950023c6a2168b229363d75f59a24ecdd66d19

New changeset 0f42b726c87f72d522893f927b4cb592b8875641 by Miss Islington (bot) in branch '3.9':
bpo-42892: fix email multipart attribute error (GH-26903) (GH-27493)
https://github.com/python/cpython/commit/0f42b726c87f72d522893f927b4cb592b8875641