Fixed #36478 -- Fixed inconsistent mail attachment handling.

Fixed an inconsistency between EmailMessage.attach() and .attachments
when attaching bytes content with a text/* mimetype. The attach()
function decodes UTF-8 bytes if possible and otherwise changes the
mimetype to application/octet-stream to preserve the content's unknown
encoding (refs #27007). Providing equivalent content directly in
EmailMessage.attachments did not apply the same logic, leading
to an "AttributeError: 'bytes' object has no attribute 'encode'"
in SafeMIMEText.set_payload().

Updated EmailMessage._create_mime_attachment() to match attach()'s
handling for text/* mimetypes with bytes content. Updated test cases
to accurately cover behavior on both paths.

1 files changed, 9 insertions, 0 deletions

diff --git a/django/core/mail/message.py b/django/core/mail/message.py
index 421e353bfa..51af560b12 100644
--- a/django/core/mail/message.py
+++ b/django/core/mail/message.py
@@ -387,6 +387,15 @@ class EmailMessage:
         email.Message or EmailMessage object, as well as a str.
         """
         basetype, subtype = mimetype.split("/", 1)
+        if basetype == "text" and isinstance(content, bytes):
+            # This duplicates logic from EmailMessage.attach() to properly
+            # handle EmailMessage.attachments not created through attach().
+            try:
+                content = content.decode()
+            except UnicodeDecodeError:
+                mimetype = DEFAULT_ATTACHMENT_MIME_TYPE
+                basetype, subtype = mimetype.split("/", 1)
+
         if basetype == "text":
             encoding = self.encoding or settings.DEFAULT_CHARSET
             attachment = SafeMIMEText(content, subtype, encoding)