fix(api): infer /documents/{id}/download Content-Type from filename when ext is omitted (#15052) (#15053)

## Summary

- Align **GET `/api/v1/documents/<doc_id>/download`** with
**`/preview`**: resolve extension and MIME type from the stored document
name when the **`ext` query parameter is omitted**, instead of
defaulting to `markdown`.
- When **`?ext=`** is present, behavior stays the same as before
(explicit extension / `Content-Type` mapping).
- Enforce the same access + document lookup pattern as preview
(**`accessible`** + **`get_by_id`**).
- Extend unit tests for the no-`ext` PDF filename case.

## Test plan

- [x] `uv run pytest
test/testcases/test_web_api/test_document_app/test_document_metadata.py::TestDocumentMetadataUnit::test_download_attachment_success_and_exception_unit`
- [x] Optional: `curl -sSI` against
`/api/v1/documents/<pdf_doc_id>/download` without `ext` and confirm
`Content-Type: application/pdf`

Fixes #15052.

api/apps/restful_apis/document_api.py

@@ -1881,10 +1881,27 @@ async def download_attachment(tenant_id=None, doc_id=None, attachment_id=None):
         # Keep backward compatibility with older callers and unit tests that still
         # pass `attachment_id` instead of the route parameter name.
         doc_id = doc_id or attachment_id
-        ext = request.args.get("ext", "markdown")
+        if not DocumentService.accessible(doc_id, current_user.id):
+            return get_data_error_result(message="Document not found!")
+
+        e, doc = DocumentService.get_by_id(doc_id)
+        if not e:
+            return get_data_error_result(message="Document not found!")
+
+        ext_arg = request.args.get("ext")
+        if ext_arg:
+            ext = ext_arg.lower().lstrip(".")
+            content_type = CONTENT_TYPE_MAP.get(ext, f"application/{ext}")
+        else:
+            m = re.search(r"\.([^.]+)$", doc.name.lower())
+            ext = m.group(1) if m else None
+            content_type = None
+            if ext:
+                fallback_prefix = "image" if doc.type == FileType.VISUAL.value else "application"
+                content_type = CONTENT_TYPE_MAP.get(ext, f"{fallback_prefix}/{ext}")
+
         data = await thread_pool_exec(settings.STORAGE_IMPL.get, tenant_id, doc_id)
         response = await make_response(data)
-        content_type = CONTENT_TYPE_MAP.get(ext, f"application/{ext}")
         apply_safe_file_response_headers(response, content_type, ext)
 
         return response

test/testcases/test_web_api/test_document_app/test_document_metadata.py

@@ -420,6 +420,11 @@ class TestDocumentMetadataUnit:
 
         # From here on the user is authorized; exercise the original branches.
         monkeypatch.setattr(module.DocumentService, "accessible", lambda _doc_id, _user_id: True)
+        monkeypatch.setattr(
+            module.DocumentService,
+            "get_by_id",
+            lambda _doc_id: (True, SimpleNamespace(name="stub.bin", type=module.FileType.OTHER.value)),
+        )
 
         async def fake_thread_pool_exec(*_args, **_kwargs):
             return b"attachment"
@@ -441,6 +446,18 @@ class TestDocumentMetadataUnit:
         assert res.headers["content_type"] == "application/abc"
         assert res.headers["extension"] == "abc"
 
+        # No `ext` query param: infer MIME/extension from the stored document filename (aligned with /preview).
+        monkeypatch.setattr(module, "request", _DummyRequest(args={}))
+        monkeypatch.setattr(
+            module.DocumentService,
+            "get_by_id",
+            lambda _doc_id: (True, SimpleNamespace(name="Annual report.PDF", type=module.FileType.PDF.value)),
+        )
+        res = _run(module.download_attachment(attachment_id="att1"))
+        assert isinstance(res, _DummyResponse)
+        assert res.headers["content_type"] == "application/pdf"
+        assert res.headers["extension"] == "pdf"
+
         async def raise_error(*_args, **_kwargs):
             raise RuntimeError("download boom")