How to Concatenate Var in dialplan?

Hi to all.

I need to concatenate a variable for file recording with mixmonitor, but not idea how to do it on dial plan. I need to end with .way file and all i get is a .raw file.

[globals]
; Support phones
$my_file=${CALLERID(num)}${CONTEXT}${DATETIME}${EPOCH}.wav|av(0)V(0)
MixMonitor($my_file)

Thanks in advance