JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file" id="image1">

JavaScript

$('#image1').change(function(event) {
    console.log('change');
    checkExtensions(this.files[0].name, $(this)); // note, I removed .get()
});

function checkExtensions (fileName, $element) {
    var allowedExtensions = new Array ('pdf','gif','jpg','png');
    var currentExtension = fileName.split('.').pop();

    if ($.inArray (currentExtension, allowedExtensions) > -1) {
        console.log('OK');
        // everything is OK, further instructions take place
    }   else {
        // reset the file input element
        console.log('Replaced');
        $element.replaceWith($element.clone(true).val(''));
    }
}