fix(api): keep empty extension whitelist as deny in CUSTOM bucket

Follow-up to the prior fix. The bucket-semantics rewrite changed the
extension-whitelist guard from `is not None` to truthiness, which
silently widened behavior for the empty-list case (UI never submits it,
but DSL / API paths could). Restore the original deny-on-empty
posture: when a file falls into the CUSTOM bucket, an explicitly set
whitelist (including []) is authoritative.

Also tightens _normalize_extension so whitespace-only input returns ""
consistent with empty input, and locks two contracts with tests:

- empty whitelist + CUSTOM bucket rejects (regression guard for the
  silent widening)
- TokenBufferMemory passes config=None to build_from_message_file
  (regression guard for the replay-skips-validation contract)

api/factories/file_factory/validation.py

@@ -8,9 +8,9 @@ from graphon.file import FileTransferMethod, FileType, FileUploadConfig
 
 
 def _normalize_extension(extension: str) -> str:
-    if not extension:
-        return ""
     s = extension.strip().lower()
+    if not s:
+        return ""
     return s if s.startswith(".") else "." + s
 
 
@@ -41,10 +41,14 @@ def is_file_valid_with_config(
     if not type_allowed and not custom_allowed:
         return False
 
+    # When the file is in the CUSTOM bucket, the extension whitelist is authoritative.
+    # An explicitly set whitelist (including the empty list) is enforced; empty == deny —
+    # the UI never submits an empty list, so this guards against DSL/API paths that
+    # bypass the UI from accidentally widening the allowlist.
     in_custom_bucket = input_file_type == FileType.CUSTOM or not type_allowed
     if (
         in_custom_bucket
-        and config.allowed_file_extensions
+        and config.allowed_file_extensions is not None
         and not _extension_matches(file_extension, config.allowed_file_extensions)
     ):
         return False

api/tests/unit_tests/core/memory/test_token_buffer_memory.py

@@ -198,6 +198,47 @@ class TestBuildPromptMessageWithFiles:
         assert isinstance(result.content[-1], TextPromptMessageContent)
         assert result.content[-1].data == "user text"
 
+    def test_replay_skips_revalidation_by_passing_config_none(self):
+        """Replay contract: history files were validated on upload, so this
+        path must call build_from_message_file with config=None. Reverting
+        this would re-trigger ENG-244 whenever workflow config drifts."""
+        conv = _make_conversation(AppMode.CHAT)
+        mem = TokenBufferMemory(conversation=conv, model_instance=_make_model_instance())
+
+        mock_file_extra_config = MagicMock()
+        mock_file_extra_config.image_config = None
+
+        real_image_content = ImagePromptMessageContent(
+            url="http://example.com/img.png", format="png", mime_type="image/png"
+        )
+        mock_app_record = MagicMock()
+        mock_app_record.tenant_id = "tenant-1"
+
+        with (
+            patch(
+                "core.memory.token_buffer_memory.FileUploadConfigManager.convert",
+                return_value=mock_file_extra_config,
+            ),
+            patch(
+                "core.memory.token_buffer_memory.file_factory.build_from_message_file",
+                return_value=MagicMock(),
+            ) as mock_build,
+            patch(
+                "core.memory.token_buffer_memory.file_manager.to_prompt_message_content",
+                return_value=real_image_content,
+            ),
+        ):
+            mem._build_prompt_message_with_files(
+                message_files=[MagicMock()],
+                text_content="user text",
+                message=_make_message(),
+                app_record=mock_app_record,
+                is_user_message=True,
+            )
+
+        mock_build.assert_called_once()
+        assert mock_build.call_args.kwargs["config"] is None
+
     @pytest.mark.parametrize("mode", [AppMode.CHAT, AppMode.AGENT_CHAT, AppMode.COMPLETION])
     def test_chat_mode_with_files_assistant_message(self, mode):
         """When files are present, returns AssistantPromptMessage with list content."""

api/tests/unit_tests/factories/test_file_validation.py

@@ -116,3 +116,27 @@ def test_history_replay_matches_round_1_outcome_under_unchanged_config():
     )
     assert _validate(input_file_type="custom", file_extension=".png", config=config) is True
     assert _validate(input_file_type="image", file_extension=".png", config=config) is True
+
+
+def test_empty_whitelist_in_custom_bucket_denies_by_default():
+    """Defensive: when a file lands in the CUSTOM bucket, an empty
+    allowed_file_extensions list rejects. The UI never submits empty;
+    this guards DSL / API paths that bypass the UI from accidentally
+    widening what's accepted."""
+    config = FileUploadConfig(
+        allowed_file_types=[FileType.CUSTOM],
+        allowed_file_extensions=[],
+    )
+    assert _validate(input_file_type="custom", file_extension=".png", config=config) is False
+    assert _validate(input_file_type="image", file_extension=".png", config=config) is False
+
+
+def test_normalize_handles_whitespace_and_empty_consistently():
+    """Whitespace-only or empty entries in the whitelist must not match real
+    extensions (regression guard for _normalize_extension edge cases)."""
+    for noisy_entry in ("", "   ", "\t"):
+        config = FileUploadConfig(
+            allowed_file_types=[FileType.CUSTOM],
+            allowed_file_extensions=[noisy_entry],
+        )
+        assert _validate(input_file_type="custom", file_extension=".png", config=config) is False