Custom Field In Opencart 2.0 Checkout Page

Need Custom Filed (File Type) In Opencart Checkout Page (No In Product Page). We will be starting medicine selling portal; As in india selling medicine prescription is required. So Need One Extra Field With Separate Step (Like Personal Details, Address) , So That Customer Can Upload A Scan Copy Or Photo Of There Prescription. This Not Be A Mandatory Filed As There Will Be Some Other Type Of Medicine Which Does Not Require prescription. This Field Should Relate To Order, Not With Customer. Admin Should View / Add / Update Each Prescription From Admin Area