diff --git a/pyrit/backend/services/converter_service.py b/pyrit/backend/services/converter_service.py
index 7ec6aec799..13e75c7152 100644
--- a/pyrit/backend/services/converter_service.py
+++ b/pyrit/backend/services/converter_service.py
@@ -196,20 +196,23 @@ async def preview_conversion_async(self, *, request: ConverterPreviewRequest) ->
                 )
                 await serializer.save_b64_image_async(data=value)
                 original_value = str(serializer.value)
-            # Already an existing file on disk — keep as-is
-            elif Path(original_value).is_file():
-                pass
             else:
-                # Treat as raw base64
-                ext = DEFAULT_MEDIA_EXTENSIONS.get(str(data_type), ".bin")
-
-                serializer = data_serializer_factory(
-                    category="prompt-memory-entries",
-                    data_type=data_type,
-                    extension=ext,
-                )
-                await serializer.save_b64_image_async(data=original_value)
-                original_value = str(serializer.value)
+                try:
+                    is_existing_file = Path(original_value).is_file()
+                except (OSError, ValueError):
+                    is_existing_file = False
+
+                if not is_existing_file:
+                    # Treat as raw base64
+                    ext = DEFAULT_MEDIA_EXTENSIONS.get(str(data_type), ".bin")
+
+                    serializer = data_serializer_factory(
+                        category="prompt-memory-entries",
+                        data_type=data_type,
+                        extension=ext,
+                    )
+                    await serializer.save_b64_image_async(data=original_value)
+                    original_value = str(serializer.value)
 
         converters = self._gather_converters(converter_ids=request.converter_ids)
         steps, final_value, final_type = await self._apply_converters_async(
diff --git a/tests/unit/backend/test_converter_service.py b/tests/unit/backend/test_converter_service.py
index 8a0d4ed03e..ceab484216 100644
--- a/tests/unit/backend/test_converter_service.py
+++ b/tests/unit/backend/test_converter_service.py
@@ -441,7 +441,7 @@ async def test_preview_conversion_persists_raw_base64_for_audio_path(self) -> No
         mock_serializer.value = "/tmp/persisted.wav"
         mock_serializer.save_b64_image_async = AsyncMock()
 
-        raw_b64 = "UklGRiQAAABXQVZF"
+        raw_b64 = "UklGRiQ" + "A" * 5000
         request = ConverterPreviewRequest(
             original_value=raw_b64,
             original_value_data_type="audio_path",
@@ -452,13 +452,77 @@ async def test_preview_conversion_persists_raw_base64_for_audio_path(self) -> No
             "pyrit.backend.services.converter_service.data_serializer_factory",
             return_value=mock_serializer,
         ) as mock_factory:
-            await service.preview_conversion_async(request=request)
+            result = await service.preview_conversion_async(request=request)
 
         mock_factory.assert_called_once()
         # ext is the audio_path mapping from DEFAULT_MEDIA_EXTENSIONS
         assert mock_factory.call_args.kwargs["extension"] == ".wav"
         assert mock_factory.call_args.kwargs["data_type"] == "audio_path"
         mock_serializer.save_b64_image_async.assert_awaited_once_with(data=raw_b64)
+        assert result.model_dump(mode="json")["original_value"] == raw_b64
+
+    async def test_preview_conversion_persists_raw_base64_when_path_inspection_fails(self) -> None:
+        """Filesystem inspection failures classify the value as raw base64."""
+        service = ConverterService()
+        mock_serializer = MagicMock()
+        mock_serializer.value = "/tmp/persisted.wav"
+        mock_serializer.save_b64_image_async = AsyncMock()
+        raw_b64 = "UklGRiQAAABXQVZF"
+        request = ConverterPreviewRequest(
+            original_value=raw_b64,
+            original_value_data_type="audio_path",
+            converter_ids=[],
+        )
+
+        with (
+            patch.object(Path, "is_file", side_effect=OSError("path inspection failed")),
+            patch(
+                "pyrit.backend.services.converter_service.data_serializer_factory",
+                return_value=mock_serializer,
+            ),
+        ):
+            result = await service.preview_conversion_async(request=request)
+
+        mock_serializer.save_b64_image_async.assert_awaited_once_with(data=raw_b64)
+        assert result.converted_value == "/tmp/persisted.wav"
+
+    async def test_preview_conversion_propagates_invalid_base64_error_after_path_failure(self) -> None:
+        """Errors from raw base64 persistence are not mistaken for path inspection failures."""
+        service = ConverterService()
+        mock_serializer = MagicMock()
+        mock_serializer.save_b64_image_async = AsyncMock(side_effect=ValueError("invalid base64"))
+        request = ConverterPreviewRequest(
+            original_value="A",
+            original_value_data_type="audio_path",
+            converter_ids=[],
+        )
+
+        with (
+            patch.object(Path, "is_file", side_effect=ValueError("invalid path")),
+            patch(
+                "pyrit.backend.services.converter_service.data_serializer_factory",
+                return_value=mock_serializer,
+            ),
+            pytest.raises(ValueError, match="invalid base64"),
+        ):
+            await service.preview_conversion_async(request=request)
+
+    async def test_preview_conversion_preserves_existing_file(self, tmp_path: Path) -> None:
+        """Existing local media paths pass through without being persisted again."""
+        service = ConverterService()
+        media_path = tmp_path / "input.wav"
+        media_path.write_bytes(b"RIFF")
+        request = ConverterPreviewRequest(
+            original_value=str(media_path),
+            original_value_data_type="audio_path",
+            converter_ids=[],
+        )
+
+        with patch("pyrit.backend.services.converter_service.data_serializer_factory") as mock_factory:
+            result = await service.preview_conversion_async(request=request)
+
+        mock_factory.assert_not_called()
+        assert result.converted_value == str(media_path)
 
 
 class TestGetConverterObjectsForIds: