diff --git a/api/controllers/service_api/dataset/rag_pipeline/rag_pipeline_workflow.py b/api/controllers/service_api/dataset/rag_pipeline/rag_pipeline_workflow.py
index 94cbee1f58..13784b2f22 100644
--- a/api/controllers/service_api/dataset/rag_pipeline/rag_pipeline_workflow.py
+++ b/api/controllers/service_api/dataset/rag_pipeline/rag_pipeline_workflow.py
@@ -10,6 +10,7 @@ from controllers.common.errors import FilenameNotExistsError, NoFileUploadedErro
 from controllers.common.schema import register_schema_model
 from controllers.service_api import service_api_ns
 from controllers.service_api.dataset.error import PipelineRunError
+from controllers.service_api.dataset.rag_pipeline.serializers import serialize_upload_file
 from controllers.service_api.wraps import DatasetApiResource
 from core.app.apps.pipeline.pipeline_generator import PipelineGenerator
 from core.app.entities.app_invoke_entities import InvokeFrom
@@ -230,12 +231,4 @@ class KnowledgebasePipelineFileUploadApi(DatasetApiResource):
         except services.errors.file.UnsupportedFileTypeError:
             raise UnsupportedFileTypeError()
 
-        return {
-            "id": upload_file.id,
-            "name": upload_file.name,
-            "size": upload_file.size,
-            "extension": upload_file.extension,
-            "mime_type": upload_file.mime_type,
-            "created_by": upload_file.created_by,
-            "created_at": upload_file.created_at,
-        }, 201
+        return serialize_upload_file(upload_file), 201
diff --git a/api/controllers/service_api/dataset/rag_pipeline/serializers.py b/api/controllers/service_api/dataset/rag_pipeline/serializers.py
new file mode 100644
index 0000000000..8533c9c01d
--- /dev/null
+++ b/api/controllers/service_api/dataset/rag_pipeline/serializers.py
@@ -0,0 +1,22 @@
+"""
+Serialization helpers for Service API knowledge pipeline endpoints.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from models.model import UploadFile
+
+
+def serialize_upload_file(upload_file: UploadFile) -> dict[str, Any]:
+    return {
+        "id": upload_file.id,
+        "name": upload_file.name,
+        "size": upload_file.size,
+        "extension": upload_file.extension,
+        "mime_type": upload_file.mime_type,
+        "created_by": upload_file.created_by,
+        "created_at": upload_file.created_at.isoformat() if upload_file.created_at else None,
+    }
diff --git a/api/tests/unit_tests/controllers/service_api/dataset/test_rag_pipeline_file_upload_serialization.py b/api/tests/unit_tests/controllers/service_api/dataset/test_rag_pipeline_file_upload_serialization.py
new file mode 100644
index 0000000000..a8dd8523ac
--- /dev/null
+++ b/api/tests/unit_tests/controllers/service_api/dataset/test_rag_pipeline_file_upload_serialization.py
@@ -0,0 +1,62 @@
+"""
+Unit tests for Service API knowledge pipeline file-upload serialization.
+"""
+
+import importlib.util
+from datetime import UTC, datetime
+from pathlib import Path
+
+
+class FakeUploadFile:
+    id: str
+    name: str
+    size: int
+    extension: str
+    mime_type: str
+    created_by: str
+    created_at: datetime | None
+
+
+def _load_serialize_upload_file():
+    api_dir = Path(__file__).resolve().parents[5]
+    serializers_path = api_dir / "controllers" / "service_api" / "dataset" / "rag_pipeline" / "serializers.py"
+
+    spec = importlib.util.spec_from_file_location("rag_pipeline_serializers", serializers_path)
+    assert spec
+    assert spec.loader
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)  # type: ignore[attr-defined]
+    return module.serialize_upload_file
+
+
+def test_file_upload_created_at_is_isoformat_string():
+    serialize_upload_file = _load_serialize_upload_file()
+
+    created_at = datetime(2026, 2, 8, 12, 0, 0, tzinfo=UTC)
+    upload_file = FakeUploadFile()
+    upload_file.id = "file-1"
+    upload_file.name = "test.pdf"
+    upload_file.size = 123
+    upload_file.extension = "pdf"
+    upload_file.mime_type = "application/pdf"
+    upload_file.created_by = "account-1"
+    upload_file.created_at = created_at
+
+    result = serialize_upload_file(upload_file)
+    assert result["created_at"] == created_at.isoformat()
+
+
+def test_file_upload_created_at_none_serializes_to_null():
+    serialize_upload_file = _load_serialize_upload_file()
+
+    upload_file = FakeUploadFile()
+    upload_file.id = "file-1"
+    upload_file.name = "test.pdf"
+    upload_file.size = 123
+    upload_file.extension = "pdf"
+    upload_file.mime_type = "application/pdf"
+    upload_file.created_by = "account-1"
+    upload_file.created_at = None
+
+    result = serialize_upload_file(upload_file)
+    assert result["created_at"] is None