IF statement with Logstash

It seems so simple, but I think i'm overlooking something.

Trying to create an IF statement in Logstash:

 if [source] == "c:/PerfLogs/test1.csv" {...}

The output which is sent to Logstash:

{"@timestamp":"2016-07-13T13:05:07.402Z","beat":{"hostname":"sen-mailmig","name":"sen-mailmig"},"count":1,"fields":null,"input_type":"log","message":"\"07/13/2016 15:05:06.833\",\"9.7147896076931755\"","offset":91261,"source":"c:/PerfLogs/Test1.csv","type":"log"}

Now Logstash skips my 'if' statement.