diff --git a/src/mistralai/models/fileschema.py b/src/mistralai/models/fileschema.py
index 7c7b60c..1994403 100644
--- a/src/mistralai/models/fileschema.py
+++ b/src/mistralai/models/fileschema.py
@@ -61,9 +61,9 @@ class FileSchema(BaseModel):
 
     @model_serializer(mode="wrap")
     def serialize_model(self, handler):
-        optional_fields = ["num_lines", "mimetype", "signature"]
-        nullable_fields = ["num_lines", "mimetype", "signature"]
-        null_default_fields = []
+        optional_fields = ("num_lines", "mimetype", "signature")
+        nullable_fields = ("num_lines", "mimetype", "signature")
+        null_default_fields = ()
 
         serialized = handler(self)
 
@@ -72,18 +72,14 @@ def serialize_model(self, handler):
         for n, f in type(self).model_fields.items():
             k = f.alias or n
             val = serialized.get(k)
-            serialized.pop(k, None)
 
             optional_nullable = k in optional_fields and k in nullable_fields
-            is_set = (
-                self.__pydantic_fields_set__.intersection({n})
-                or k in null_default_fields
-            )  # pylint: disable=no-member
+            is_set = n in self.__pydantic_fields_set__ or k in null_default_fields
 
             if val is not None and val != UNSET_SENTINEL:
                 m[k] = val
             elif val != UNSET_SENTINEL and (
-                not k in optional_fields or (optional_nullable and is_set)
+                k not in optional_fields or (optional_nullable and is_set)
             ):
                 m[k] = val