proxyReqBodyDecorator with multipart form data

[praneetha-ck-robo]

How to use proxyReqBodyDecorator when dealing with multipart formData?
If it would be a normal json request, the following code would work. But when dealing with formData inside the proxy, how do you modify and send the same?

app.use('/proxy', proxy('www.google.com', {
  proxyReqBodyDecorator: function(bodyContent, srcReq) {
    return {
         ... bodyContent,
         'key1':srcReq. session.value1you edi
         'key2':srcReq. session.value2
    }
  }
}));