UX for uploading documents in long form flow

I have a government client using Pega to control the flow of a business case where a user flows through multiple form screens. Within several of these forms, the user is prompted to optionally upload supporting documents - optional but desired by the business for data gathering. There is disagreement about whether the user should be prompted to upload all documents at the end of the flow from a single view, or add the upload prompt to each screen where the context is provided. I am interested to hear if anyone has opinion on these options.