Adding file uploads to commerce product line items?
You will not be able to upload an asset into a line item at Commerce 1 or 2.

You only 2 options are 

1) Upload the asset to an order custom field(s). Submit a new question with your code if this is not working for you.

2) Upload the asset anywhere else, (including optionally the order), and then put the URL of the asset into the line items options data. See https://docs.craftcms.com/commerce/v2/adding-to-and-updating-the-cart.html#line-item-options-and-notes