Resolution:
The FCP channel is used between the Interior Server and the DMZ for transferring large file contents exceeding a given file size threshold (set in the "Skip Content Threshold (KB)" of the Intercomponent Advanced screen of the BC deployment configuration) .  

The FCP channel is configured under the "File Copy Server Host:Port" section of the same screen.  This setting will work if there is only one DMZ Server.  When using multiple DMZ servers, you must add the following to each DMZ tra file:

tibco.clientVar.gatewayProperties/server/fcpHostPort=&lthostname>\:&ltport number>

where "hostname" is the DNS name or IP address of the server that the DMZ is located on, and &ltport number> is the same TCP port number that is configured in the GUI, and must be the same for all DMZ servers.  The host parameter gets sent to the Interior Server by the DMZ, and it tells the interior server which host has sent the inbound request.  If the port number is changed in the GUI, all TRA files must be updated with the same TCP port number.

Configuring FCP when using Multiple DMZ Servers