It has been a long pending demand of customers, users, partners and developers to add lookup field of Document Type or Category in Upload or Edit Properties of SharePoint Document in Dynamics 365 CRM and Portal to identify a document such as Passport, Identity Card, Photo, Degree Certificate etc.
In on-premises D365 CRM there were options to add custom fields in SystemBaseForm entity by modifying the FormJson and FormXml but in online CRM 365 SystemForm entity doesn't allow any update or modification neither through XRMToolbox nor through WebAPI.
Could you please suggest any work around to customize it to add this missing capability without using any 3rd party production or solution.