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)
diff --git a/tests/mail/tests.py b/tests/mail/tests.py
index 301e589409..b0fef71e98 100644
--- a/tests/mail/tests.py
+++ b/tests/mail/tests.py
@@ -901,12 +901,39 @@ class MailTests(MailTestsMixin, SimpleTestCase):
                     self.assertEqual(actual.mimetype, expected_mimetype)
 
     def test_attach_text_as_bytes(self):
-        msg = EmailMessage()
-        msg.attach("file.txt", b"file content\n")
-        filename, content, mimetype = self.get_decoded_attachments(msg)[0]
-        self.assertEqual(filename, "file.txt")
-        self.assertEqual(content, "file content\n")
-        self.assertEqual(mimetype, "text/plain")
+        """
+        For text/* attachments, EmailMessage.attach() decodes bytes as UTF-8
+        if possible and changes to DEFAULT_ATTACHMENT_MIME_TYPE if not.
+        """
+        email = EmailMessage()
+        # Mimetype guessing identifies these as text/plain from the .txt extensions.
+        email.attach("utf8.txt", "ütƒ-8\n".encode())
+        email.attach("not-utf8.txt", b"\x86unknown-encoding\n")
+        attachments = self.get_decoded_attachments(email)
+        self.assertEqual(attachments[0], ("utf8.txt", "ütƒ-8\n", "text/plain"))
+        self.assertEqual(
+            attachments[1],
+            ("not-utf8.txt", b"\x86unknown-encoding\n", "application/octet-stream"),
+        )
+
+    def test_attach_text_as_bytes_using_property(self):
+        """
+        The logic described in test_attach_text_as_bytes() also applies
+        when directly setting the EmailMessage.attachments property.
+        """
+        email = EmailMessage()
+        email.attachments = [
+            ("utf8.txt", "ütƒ-8\n".encode(), "text/plain"),
+            ("not-utf8.txt", b"\x86unknown-encoding\n", "text/plain"),
+        ]
+        attachments = self.get_decoded_attachments(email)
+        self.assertEqual(len(attachments), 2)
+        attachments = self.get_decoded_attachments(email)
+        self.assertEqual(attachments[0], ("utf8.txt", "ütƒ-8\n", "text/plain"))
+        self.assertEqual(
+            attachments[1],
+            ("not-utf8.txt", b"\x86unknown-encoding\n", "application/octet-stream"),
+        )
 
     def test_attach_utf8_text_as_bytes(self):
         """