firelab-general/static/js/img_preview.js

let imgPreview = document.getElementById('img-preview');
let imgUpload = document.getElementById('img-upload');
imgUpload.onchange = event => {
    const [file] = imgUpload.files;
    if (file) {
        console.log(file)
        imgPreview.src = URL.createObjectURL(file);
    }
};