If you try to send a submission that has media/attachments that are above (by size) the limit set in NGINX/Central, then the submission will fail and it will be stuck in "ready to send":
the submission is still sent to Central, minus the too large media/attachment, in fact it will show in Central GUI with a "missing attachment" warning.
In the meantime in Briefcase the submission is stuck there, there is no way to remove it from the within the App, the submission is already in a state that cannot be edited and the user is left with no information that actually it has already been sent minus the problematic media/attachment.