Based on MC204541 dated 24/02/2021 (see below but now no longer available somehow), it appears that this feature was planned for 2021 Release Wave 1 but it must have been pulled out at the 11th hour:

Message Summary
Customer Voice is introducing support for file attachments in a survey. Files uploaded by survey respondents are uploaded to the Dataverse and stored in Azure blob storage. In order to support this capability, we are adding a new entity named "Customer Voice file response" (msfp_fileresponse) to your Dataverse instance.

How will this affect me?
No action is needed. This entity change is supporting the upcoming Customer Voice functionality and you can use the same Project owner role to control who can create surveys and generate survey response records with associated file responses.

If you have any questions about the new functionality, you can contact the Customer Voice team at customervoice@microsoft.com.

Thank you for being a valued customer,
Dynamics 365 Customer Voice product team