Short answer
HubSpot's KB: a File property stores files associated with a record. Each can contain up to 10 files. On records the upload limit is 20 MB with free tools and 50 MB with paid subscriptions; through a form it's 100 MB per file. You choose who can view the files when creating the property.
1. The field type
Creating a property in my demo portal.

2. From forms
See file upload fields.