Issue Summary

When you have a transition screen with the Attachment field in it and add this screen to a workflow transition, if Enhanced Board is enabled and you try to add an attachment while doing the transition, you will get this error "Upload in progress, please wait" when trying to complete the transition.

Steps to Reproduce

1. Create a company-managed project, I choose Kanban project type.
2. Create a screen and add the Attachment field to it.
3. Add this screen to a transition. For example, I added the screen in All > Done transition.
4. Enable Enhanced Board. 
5. It won't reproduce the first time, but you can try the transition a couple of times and cancel it.
6. Refresh your browser page, keep trying the transition, at some point after 3 or 4 tries, you will end up with this error when clicking Done to finish the transition.

Expected Results

Attachment file is uploaded and shows up in the transition screen and once clicking Done transition is completed successfully.

Actual Results

The attachment is not added to the screen, when you try clicking Done, it will show this error message: "Upload in progress, please wait" and block you from finishing the transition.

Workaround

Disable Enhanced Board and retry the transition.