Add custom JSON encoder to handle attachment objects

atst/utils/json.py

@@ -0,0 +1,9 @@
+from flask.json import JSONEncoder
+from atst.models.attachment import Attachment
+
+
+class CustomJSONEncoder(JSONEncoder):
+    def default(self, obj):
+        if isinstance(obj, Attachment):
+            return obj.filename
+        return JSONEncoder.default(self, obj)