make sure max_length is in FileField kwargs

rest_framework/utils/field_mapping.py

@@ -192,7 +192,8 @@ def get_field_kwargs(field_name, model_field):
     # rather than as a validator.
     max_length = getattr(model_field, 'max_length', None)
     if max_length is not None and (isinstance(model_field, models.CharField) or
-                                   isinstance(model_field, models.TextField)):
+                                   isinstance(model_field, models.TextField) or
+                                   isinstance(model_field, models.FileField)):
         kwargs['max_length'] = max_length
         validator_kwarg = [
             validator for validator in validator_kwarg