I am able to retrieve attachments from contact object but I can't view that attachment because its by default hidden true. How I can retrieve those files without uploading in file manager?
Files are attached as note engagements on the contact and would be returned in this API and then get the file metadata from there. One thing to note though is that if a file was uploaded outside of the file manager in HubSpot, i.e. attached directly on the record, then it will show as hidden:true and you can not get the file metadata via API.