Hi @fdemo
The reason for this that the file manager and object attachments are completely seperate stores of data within HubSpot. The API for the file manager allows the upload of files to the file manager (which is intended for hosting web content) whereas there is no API (as far as I am aware) to add an attachement to a CRM object - currently.
Workarounds to allow the uploading of a file via API and ‘attaching’ it to a CRM object have been discussed, for example: https://community.hubspot.com/t5/APIs-Integrations/Attach-file-to-deal-record-using-API/m-p/719945#M58797
Hope this helps.