Add a special serializer class for FileField which has a custom to_representation function

- This was solving a very subtle bug which will probably only ever apply to a single installation instance
- Future me will most likely not remember what this was for or how it works
- In any case, there we go
- Ref: http://www.cdrf.co/3.9/rest_framework.fields/Field.html

(cherry picked from commit 7305094854)

InvenTree/stock/serializers.py

@@ -15,6 +15,7 @@ from django.db.models.functions import Coalesce
 from company.serializers import SupplierPartSerializer
 from part.serializers import PartBriefSerializer
 from InvenTree.serializers import UserSerializerBrief, InvenTreeModelSerializer
+from InvenTree.serializers import InvenTreeAttachmentSerializerField
 
 
 class LocationBriefSerializer(InvenTreeModelSerializer):
@@ -232,6 +233,8 @@ class StockItemTestResultSerializer(InvenTreeModelSerializer):
 
     key = serializers.CharField(read_only=True)
 
+    attachment = InvenTreeAttachmentSerializerField()
+
     def __init__(self, *args, **kwargs):
         user_detail = kwargs.pop('user_detail', False)