1. You can incorporate the DMZedge Server outside the firewall and then use Cornerstone MFT Server inside the firewall. This solution would bypass NAT completely as all traffic would occur between the Cornerstone server and the DMZedge Server
  2. You can disable non-secure FTP and simply use FTPS and enable the 'External IP Address' feature in Cornerstone. This would allow Cornerstone to perform the proper NAT over the SSL encrypted channel (since NAT does not work over SSL). The downside is that he would not be able to use standard FTP.
  3. You can use SFTP (Secure FTP over SSH) which is more secure than FTP and faster than FTPS and would bypass NAT completely.