JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" enctype="multipart/form-data" action="">
    <label for="fname">First Name:</label>
    <input type="text" name="fname" id="teste" />
    <br />
    <br />
    <label for="file">Select File:</label>
    <input type="file" name="file" />
    <input type="submit" name="submit" value="Submit" />
</form>

JavaScript

window.onload = (function () {
    document.forms[0].addEventListener('submit', function (evt) {
        var file = document.forms[0].file;
        if (file && file.size < 18000) {
            //Submit form
            alert('Size is valid');
        } else {
            alert('pic too big');
            evt.preventDefault();
        }
    }, false);
});