Design Mockups, etc docs that are attached to a To Do should be able to be promoted/moved to the release level without the pain of downloading and uploading the document
Thank you for the request. There are a couple workarounds which can help with communication (including adding a hyperlink to the release with the mockup). Alternatively, you might consider asking the assignee to attach the mockup directly to the release level.
At this time, we do not have plans for updates in this area based on current priorities. But we hope that some of the suggestions above can help with the workflow.