add shim for legacy mixed_subtype

Author: peterfarrell

src/herald/base.py

@@ -4,6 +4,7 @@
 from mimetypes import guess_type
 
 import jsonpickle
+import django
 from django.conf import settings
 from django.contrib.sites.models import Site
 from django.core.files import File
@@ -14,6 +15,10 @@
 
 from herald.utils import get_sent_notification_model
 
+use_legacy_mixed_subtype: bool = False
+if django.VERSION < (5, 0):
+        use_legacy_mixed_subtype: bool = True
+
 
 class NotificationBase:
     """
@@ -451,7 +456,8 @@ def _send(
             # All mimebase attachments must have a Content-ID or Content-Disposition header
             # or they will show up as unnamed attachments"
             if isinstance(attachment, MIMEBase):
-                if attachment.get("Content-ID", False):
+                # `mixed_subtype` was removed from Django 5+
+                if use_legacy_mixed_subtype and attachment.get("Content-ID", False):
                     # if you are sending attachment with content id,
                     # subtype must be 'related'.
                     mail.mixed_subtype = "related"