api/rest.py,core/models.py: WIP support for attachement access

WIP draft of support for accessing attachments.

squad/api/rest.py

@@ -1310,9 +1310,17 @@ class StatusViewSet(NestedViewSetMixin, ModelViewSet):
 
 class AttachmentSerializer(serializers.ModelSerializer):
 
+    attachment_url = serializers.SerializerMethodField()
+
     class Meta:
         model = Attachment
-        fields = ('filename', 'mimetype', 'length')
+        fields = '__all__'
+
+    def get_attachment_url(self, attachment):
+        request = self.context.get('request')
+        if request is None:
+            return None
+        return request.build_absolute_uri(f"/{attachment.test_run.project}/build/{attachment.test_run.build.version}/attachments/testrun/{attachment.pk}/{attachment.filename}")
 
 
 class TestRunSerializer(DynamicFieldsModelSerializer, serializers.HyperlinkedModelSerializer):