I know you already support the ability to attach an additional document to your submission emails. The limitation to this is that the same document is attached every single time. The mobile user has no control over it.
This would be the ability for mobile users to choose various documents to attach to the same email the submission PDF is attached to. Documents would be uploaded via the Canvas website by an Admin similar to how reference data files or reference images are uploaded. Mobile users would then see the items listed on their mobile device and could select the ones they want to attach.