Update app.py

app.py

@@ -21,7 +21,7 @@ class AttachmentDownloadTool(Tool):
     output_type = "bytes"
 
     def forward(self, task_id):
-        download_url = f"{api_url}/files/"
+        download_url = f"{DEFAULT_API_URL}/files/"
         
         try:
             response = requests.get(download_url + task_id, stream=True, timeout=15)