I don’t understand why are you trying to set the FILENAME variable. That too you have given a path to a wav file in the /var/spool/asterisk/outgoing directory. This line makes no sense. You must not write anything other than asterisk .call files in this directory. Remove this line.
In MixMonitor, specify absolute path to the recording with the extension eg. /home/recordings/.wav where in place of file name give proper name or replace this with a variable containing the name of the file. To choose a unique file name you can use ${UNIQUEID} variable which is the unique id for every call. so you can do something like MixMonitor(/home/recordings/${UNIQUEID}).