app.js

function handleFileSelect(evt) {

    var files = evt.target.files; // FileList object

    // Loop through the FileList
    for (var i = 0, f; f = files[i]; i++) {

        // Only process image files
        if (!f.type.match('image.*')) {
            continue;
        }

        var reader = new FileReader();

        // Closure to capture the file information
        reader.onload = (function(theFile) {
            return function(e) {
            // Render thumbnail.
            var span = document.createElement('span');
            span.innerHTML = ['<img class="thumb" src="', e.target.result,
                                '" title="', escape(theFile.name), '"/>'].join('');
            document.getElementById('preview').insertBefore(span, null);
            };
        })(f);

        // Read in the image file as a data URL.
        reader.readAsDataURL(f);
    }
}

document.getElementById('file').addEventListener('change', handleFileSelect, false);