Onchange event based on multi-file upload

I am working on a form that currently has a standard File Upload field for a single file. I have a javascript function triggered by the onchange event on that field that is working well. However, I am looking to implement that same function on a File Upload field that has multi-file turned on and I can’t get the event to trigger. I’m assuming that the element ID I need to watch for that event on is something different than input_{form id}_{field id}, but I can’t figure out what it needs to be. Has anybody had any experience with anything like this?

Here is the javascript as it currently works for the single-file upload:

document.getElementById('input_3_15').onchange = setFileInfo;
function setFileInfo() {

function readFileDuration(file){
    return new Promise((resolve, reject)=>{
        //var type = mime.split("/")[0];
        var type = 'video';
        var video = document.createElement(type);
        video.preload = 'metadata';
        video.onloadedmetadata = function() {
            //When the onloadedmetadata function responds, we execute the following
	        if (isNaN(video.duration) == false) {
                //this is the moment where the readFileDuration return a value
        video.src = URL.createObjectURL(files[i]);;

let promisesArray = [];
var counter = 0;
var files = this.files;
var secs = 0;

//We create an array of promises
for (i=0; i < files.length; i++) {

//We instruct to execute all promises
Promise.all(promisesArray).then(results =>{
    /* this code only executes when all promises have resolved their value.
      The result is an array with all returned values
    for (i=0; i < results.length; i++) {
        secs += results[i];
    var mins = Math.ceil(secs / 60);