fix: handle single file in cases of multiple file uploads (#2950)

* fix: correctly parse files when uploading list of files

When a model is specified for uploading files and a specific field in that model is annotated with
`list[UploadFile]` and a single file is uploaded, the validation fails. This is because the extracted
file is not passed onto `msgspec` as a list. This fixes that by ensuring that the value is converted
into a list if needed before giving it to `msgspec` for validation.

* fix: handle case where name may not be in the form data

If there was no field name with the expected name (as per the given model), we don't want to raise a KeyError there.
Instead the missing field name should be raised as a validation error by the validation library.

* refactor: use API provided by FieldDefinition

litestar/_kwargs/extractors.py

@@ -15,6 +15,7 @@
 from litestar.exceptions import ValidationException
 from litestar.params import BodyKwarg
 from litestar.types import Empty
+from litestar.utils.predicates import is_non_string_sequence
 from litestar.utils.scope.state import ScopeState
 
 if TYPE_CHECKING:
@@ -355,7 +356,7 @@ async def extract_multipart(
 
         if field_definition.is_non_string_sequence:
             values = list(form_values.values())
-            if field_definition.inner_types[0].annotation is UploadFile and isinstance(values[0], list):
+            if field_definition.has_inner_subclass_of(UploadFile) and isinstance(values[0], list):
                 return values[0]
 
             return values
@@ -366,7 +367,15 @@ async def extract_multipart(
         if not form_values and is_data_optional:
             return None
 
-        return data_dto(connection).decode_builtins(form_values) if data_dto else form_values
+        if data_dto:
+            return data_dto(connection).decode_builtins(form_values)
+
+        for name, tp in field_definition.get_type_hints().items():
+            value = form_values.get(name)
+            if value is not None and is_non_string_sequence(tp) and not isinstance(value, list):
+                form_values[name] = [value]
+
+        return form_values
 
     return cast("Callable[[ASGIConnection[Any, Any, Any, Any]], Coroutine[Any, Any, Any]]", extract_multipart)
 

tests/unit/test_kwargs/test_multipart_data.py

@@ -410,6 +410,27 @@ async def handler(data: List[UploadFile] = Body(media_type=RequestEncodingType.M
         assert response.status_code == HTTP_201_CREATED
 
 
+@dataclass
+class Files:
+    file_list: List[UploadFile]
+
+
+@pytest.mark.parametrize("file_count", (1, 2))
+def test_upload_multiple_files_in_model(file_count: int) -> None:
+    @post("/")
+    async def handler(data: Files = Body(media_type=RequestEncodingType.MULTI_PART)) -> None:
+        assert len(data.file_list) == file_count
+
+        for file in data.file_list:
+            assert await file.read() == b"1"
+
+    with create_test_client([handler]) as client:
+        files_to_upload = [("file_list", b"1") for _ in range(file_count)]
+        response = client.post("/", files=files_to_upload)
+
+        assert response.status_code == HTTP_201_CREATED
+
+
 def test_optional_formdata() -> None:
     @post("/", signature_types=[UploadFile])
     async def hello_world(data: Optional[UploadFile] = Body(media_type=RequestEncodingType.MULTI_PART)) -> None: