How to add id property in FormData object? (Javascript)

This is my function for uploading files:

var fd = new FormData();
fd.append("file", $scope.files[i]);


$http.post('/api/images/', fd, {
    withCredentials: true,
    headers: {'Content-Type': undefined },
    transformRequest: angular.identity
})
.success(function(data) {
    console.log(data);
})
.error(function(data) {
    console.log('Error: ' + data);
});

I want to send also id with this file. I think i must add this:

fd.append("id", id);

But how i can read this on the server side? For example, when i want a name of file i write this

app.post('/api/images/', function(req, res) {
    console.log(req.files.file.name); 
});

But this doesn't work:

console.log(req.id);

I'm using node.js with express.