Uploading documents to a deal in hubspot

I want to implement the following use case and I'm not sure how to do this with the Hubspot API and I don't find any solution within the documentation. I created a custom request flow which is creating contacts and deals within Hubspot programmatically via the API in the next step the user should upload documents related to his case and I would like to upload and associate these documents with the related deal. Could someone help me or point me to the right spot in the documentation on how such a scenario can be established with the API