Update app.py

app.py

@@ -26,12 +26,12 @@ class AttachmentDownloadTool(Tool):
         try:
             response = requests.get(download_url + task_id, stream=True, timeout=15)
             if response.status_code != 200:
-                return None
+                return io.BytesIO()
             file_obj = io.BytesIO(response.content)
             file_obj.seek(0)
             return file_obj
         except Exception as e:
-            return None
+            return io.BytesIO()
 
 class ImageCaptionTool(Tool):
     name = "image-captioner"