JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file" id="fileUpload"/>

JavaScript

document.getElementById("fileUpload").onchange = function() {
    var fileName = this.value;
    var fileExtension = fileName.substr(fileName.length - 4);
    
    console.log(fileExtension)
    if (fileExtension != ".xls") {
        alert("That ain't no .xls file!");
    }
}