Hi A-E-Albert, the feature to add an attachment from the VAT returns page remains unavailable in QuickBooks Online. If you have the bank account connected, you can add the attachment to the transaction in the For-Review tab before matching this to the payment that you have recorded from the Taxes screen.
To add the attachment on the for-review screen, click into the transaction to edit and select 'Add Attachment' then select the document to upload on the pop-up window. Once this is attached you can then select 'Match' to move this into your reviewed transactions.