Currently our system does not support this. The leave needs to be cancelled or rejected back to the applicant for them to attach the MC.
You are able to configure the leave type such that attachments are made mandatory. Go to eLeave > Leave Setup > Leave Types and enable the 'Is Attachment Mandatory' option, and click 'update'.
After that is done, employees would need to provide an attachment before they are able to submit the leave application successfully.