Ability to add files to custom object

We have an inventory object and we need to attach calibration and certification files that need to be tied to the asset.  Tied to the asset regardless of who the current contact or company is as the products can be transferred between contact and sold to different companies.  Using the file field for contact does not work.... 

The key concept here is that people and companies are transient to a product's users and ownership. My wife can bring my car to the dealership for service. Even though she brought it in, the work is still tied to the asset which I own.  Even if I sell the vehicle, the work history (past and present) is still tied to the s/n of the car.  In our scenario, I need to attach files at key points of the product.