zabbix_sender processes key names from -k option and from file differently. With -k option it is possible to send spaces in key names:

Example:

zabbix_sender -c /etc/zabbix/zabbix_agentd.conf -vv -s testhost1 -k 'monitor[test,"Test One Two Three",value]' -o 123 

When using the same value in file with -i option it gets parsed incorrectly (snip from server log):

{"host":"testhost1","key":"'monitor[test\"Test","value":"Oen Two Three\",test]' 123"}