We have also come across this issue, unfortunately there is no 'Add and Review' button anymore which is what allowed one to do this in the past. It seems attachments cannot be added until a requisition exists for that request (i.e. a valid requisition number exists).
There were some small changes we did to make the flow a little simpler though, we modified the order confirmation page to included a hyper link to navigate back to the request without having to go back to ongoing orders, this was done by modifying the handlebar templates and scripts so it can be a complex change.