Node.js maxFieldSize exceeded

I am uploading an array of images to my node.js express server, but I'm getting an error - "Error: maxFieldsSize exceeded". It looks like the default maxFieldsSize is 2MB. I need to be able to upload up to 15MB of data. How do I increase the maxFiedsSize of my MultipartParser?

EDIT:

exports.perform = function(req, res) {
incomingForm.maxFieldsSize = 20 * 1024 * 1024; // increase fields size?
var json = req.body;
var nodemailer = require('../node_modules/nodemailer');
var fs = require('fs');
var exec = require('child_process').exec;
var events = require('events');
var mustache = require('../html/mustache');

// build html
var buildHtml = function(error) {

    console.log("building html");       

    var html = fs.open('./temp.html', 'w');
    var strTemplate;
    if (json.user_language == 'de')
        strTemplate = fs.readFileSync('./email/RM01_CLAIM_FORM_DE.html').toString();
    else
        strTemplate = fs.readFileSync('./email/RM01_CLAIM_FORM_EN.html').toString();

    // get data
    var data = {
        passenger_name : json.first_name + ' ' + json.last_name,
        passenger_address : json.passenger_address,
        passenger_telephone : json.passenger_telephone,
        passenger_email : json.passenger_email,
        flight_info : json.flight_info,
        ticket_number : json.ticket_number,
        complaint_reason : json.complaint_reason,
        extraordinary_circumstances : json.extraordinary_circumstances,
        legal_insurance : json.legal_insurance,
        comments : json.comments,
        language : json.user_language,
        media : json.media_attachments
    }

...