这是我的javascript
upload.js文件代码：

var handleUpload = function(event) {
    event.preventDefault();
    event.stopPropagation();

    var fileInput = document.getElementById('image_id');

    var data = new FormData();

    data.append('javascript',true);

        if(fileInput.files[0].size > 1050000) {
        document.getElementById("image_id").innerHTML = "Image too big (max 1Mb)";
        alert('Fotografija koju želite dodati je veća od 1Mb!');
        window.location="upload_images.PHP"
        return false;
        }

    for (var i =0; i < fileInput.files.length; ++i) {
        data.append('image',fileInput.files[i]);
    }

    var request = new XMLHttpRequest();

    request.upload.addEventListener('progress',function(event) {
        if (event.lengthComputable) {
            var percent = event.loaded / event.total;
            var progress = document.getElementById('upload_progress');

            while (progress.hasChildNodes()) {
                progress.removeChild(progress.firstChild);
            }

            progress.appendChild(document.createTextNode(Math.round(percent * 100) + ' %'));
        }
    });

    request.upload.addEventListener('load',function(event) {
        document.getElementById('upload_progress').style.display = 'none';
    });

    request.upload.addEventListener('error',function(event) {
        alert('Dodavanje slika nije bilo uspješno! Pokušajte ponovo.');
    });

    request.addEventListener('readystatechange',function(event) {

        if (this.readyState == 4) {
            if(this.status == 200) {
                var links = document.getElementById('uploaded');

                window.location="upload_images.PHP?success"
                console.log(this.response); 
            } else {
                console.log('Server replied with HTTP status ' + this.status);
            }
        }

    });

    request.open('POST','upload_images.PHP');
    request.setRequestHeader('Cache-Control','no-cache');

    document.getElementById('upload_progress').style.display = 'block';

    request.send(data);
}

window.addEventListener('load',function(event) {
    var submit = document.getElementById('submit');
    submit.addEventListener('click',handleUpload);

});

这是我上传文件的形式：

<div id="uploaded">

</div>
<div>
<form action="" method="post" enctype="multipart/form-data">
<input name="image" id="image_id" type="file" size="25" value="Odaberi sliku" />
<input type="submit" id="submit" value="Dodaj Foto"/>
</form>
</div>
<div class="upload_progress" id="upload_progress"></div>

我需要在那个javascript代码中检查是文件是图像.允许jpg,jpeg,png和gif扩展名并阻止其他的.提醒用户他是否尝试上传其他类型的文件.