Synaccess Networks NP-0801DTM - Manual (Page 253)

FTP AT COMMANDS
253 HSPA+ AT Commands Reference Guide
// Note: If while sending the chunks the data port is closed from remote, user will be aware of it because #FTPAPPEXT
indicates ERROR and cause (available if previously issued the command AT+CMEE=2) will indicate that socket has been
closed. Also in this case obviously, data port must be reopened with FTPPUT and so on…(same sequence)