How to change the file which I senh as FormData

I create file in my local storage using file-service. I use FormData to send my File.
This is audio and I want change it’s format using some node-libraries. To do it I need to get this File object in pre-process server side script,but I don’t know how can I do it.
This is my “event” object.
io-4433-2017-12-04-16-25-25-062