fix(email): thread attachment sends on the caller's reply_to

send_document() accepted reply_to but never passed it down, so attachments
always threaded from the cached per-address context (or not at all) even
when the caller named the message to reply to. The plain-text path
(_send_email) already honored it; the attachment path now does too.

The metadata half of #10131 (send_image rejecting metadata=) was already
fixed on main by the adapter parity pass. Diagnosis from #10131 and the
explicit-reply_to-wins shape from PR #10321 (which targeted the
pre-plugin path).

Fixes #10131
Co-authored-by: LeonSGP43 <cine.dreamer.one@gmail.com>

plugins/platforms/email/adapter.py

@@ -703,9 +703,11 @@ class EmailAdapter(BasePlatformAdapter):
         logger.info("[Email] Sent reply to %s (subject: %s)", to_addr, subject)
         return msg_id
 
-    def _send_with_files(self, to_addr: str, body: str, files: List[Tuple[Path, str]], *, lenient: bool) -> str:
-        """Send a reply with attachments; *lenient* logs-and-skips unattachable files instead of raising."""
-        msg, msg_id, _ = self._new_reply(to_addr, body)
+    def _send_with_files(self, to_addr: str, body: str, files: List[Tuple[Path, str]], *, lenient: bool,
+                         reply_to_msg_id: Optional[str] = None) -> str:
+        """Send a reply with attachments; *lenient* logs-and-skips unattachable files instead of raising.
+        An explicit *reply_to_msg_id* threads the mail like ``_send_email`` does (#10131)."""
+        msg, msg_id, _ = self._new_reply(to_addr, body, reply_to_msg_id)
         for path, name in files:
             try:
                 _attach_file(msg, path, name)
@@ -755,11 +757,14 @@ class EmailAdapter(BasePlatformAdapter):
     async def send_document(self, chat_id: str, file_path: str, caption: Optional[str] = None,
                             file_name: Optional[str] = None, reply_to: Optional[str] = None, **kwargs) -> SendResult:
         """Send a file as an email attachment."""
-        return await self._run_send(self._send_email_with_attachment, (chat_id, caption or "", file_path, file_name), "[Email] Send document failed: %s")
+        return await self._run_send(self._send_email_with_attachment, (chat_id, caption or "", file_path, file_name, reply_to),
+                                    "[Email] Send document failed: %s")
 
-    def _send_email_with_attachment(self, to_addr: str, body: str, file_path: str, file_name: Optional[str] = None) -> str:
+    def _send_email_with_attachment(self, to_addr: str, body: str, file_path: str, file_name: Optional[str] = None,
+                                    reply_to_msg_id: Optional[str] = None) -> str:
         """Send an email with a single file attachment via SMTP (raises if unattachable)."""
-        return self._send_with_files(to_addr, body, [(Path(file_path), file_name or Path(file_path).name)], lenient=False)
+        return self._send_with_files(to_addr, body, [(Path(file_path), file_name or Path(file_path).name)], lenient=False,
+                                     reply_to_msg_id=reply_to_msg_id)
 
     async def get_chat_info(self, chat_id: str) -> Dict[str, Any]:
         """Return basic info about the email chat."""

tests/gateway/test_email.py

@@ -432,6 +432,27 @@ class TestSendMethods(unittest.TestCase):
             os.unlink(tmp_path)
 
 
+    def test_send_document_threads_on_explicit_reply_to(self):
+        """An explicit reply_to wins over the cached thread context for attachment sends (#10131)."""
+        import asyncio
+        import tempfile
+        adapter = self._make_adapter()
+        adapter._thread_context["user@test.com"] = {"subject": "Old", "message_id": "<cached@test.com>"}
+        with tempfile.NamedTemporaryFile(suffix=".txt", delete=False) as f:
+            f.write(b"doc")
+            tmp_path = f.name
+        try:
+            with patch("smtplib.SMTP") as mock_smtp:
+                mock_server = MagicMock()
+                mock_smtp.return_value = mock_server
+                result = asyncio.run(adapter.send_document("user@test.com", tmp_path, reply_to="<explicit@test.com>"))
+                self.assertTrue(result.success)
+                sent_msg = mock_server.send_message.call_args[0][0]
+                self.assertEqual(sent_msg["In-Reply-To"], "<explicit@test.com>")
+                self.assertEqual(sent_msg["References"], "<explicit@test.com>")
+        finally:
+            os.unlink(tmp_path)
+
     def test_get_chat_info(self):
         """get_chat_info should return email address as chat info."""
         import asyncio