Google chat app is not responding for image attachment - google-apps-script

Question

I am trying to save attachment uploaded by google chat app.
I am handling two cases if user messages anything to google chat app, it will pop up a card with widgets which is perfectly fine but I am handling if user uploads attachment it will save into google drive but the code is not working. Can someone please help?
I am new in coding.
function getByteStream(dataRef) {
var blob = "";
var driveFileName = "";
var url = "https://chat.googleapis.com/v1/media/"+ dataRef +"?alt=media"
var service = getOAuth2Service();
var response = UrlFetchApp.fetch(url, {
headers: {
'Authorization': 'Bearer ' + service.getAccessToken(),
},
'muteHttpExceptions': true,
});
if(response.getResponseCode() != 200){
return "Failed to get file content with error code: " + response.getResponseCode();
}
blob = response.getBlob();
driveFileName = DriveApp.createFile(blob);
return "The uploaded contents have been saved in a Google Drive file: "+ driveFileName;
}
function onMessage(event) {
if (event.message.attachment) {
var attachmentName = event.message.attachment[0].name;
var attachment = getByteStream(attachmentName);
console.log(attachment);
} else {
return {
"cards_v2": [{
"card_id": "addContact",
"card": {
"header": {
"title": "Hello" +" " + event.user.displayName + "!",
"subtitle": "Log your feedback!",
"imageUrl": "https://www.gstatic.com/images/branding/product/2x/contacts_48dp.png",
"imageType": "CIRCLE"
},
"sections": [
{
"widgets": [
{
"buttonList": {
"buttons": [
{
"text": "Start here",
"onClick": {
"action": {
"function": "openDialog",
"interaction": "OPEN_DIALOG"
}
}
}
]
},
"horizontalAlignment": "CENTER"
}
]
}
]
}
}]
};
}
}