JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file">

JavaScript

$('input').on('change', function() {
    alert('Nome: ' + this.files[0].name + '\nTamanho do ficheiro: ' + humanFileSize(this.files[0].size));

});


function humanFileSize(bytes, si) {
    var thresh = si ? 1000 : 1024;
    if(bytes < thresh) return bytes + ' B';
    var units = si ? ['kB','MB','GB','TB','PB','EB','ZB','YB'] : ['KiB','MiB','GiB','TiB','PiB','EiB','ZiB','YiB'];
    var u = -1;
    do {
        bytes /= thresh;
        ++u;
    } while(bytes >= thresh);
    return bytes.toFixed(1)+' '+units[u];
};