来自数据blob的文件扩展名

我有javascript代码检测图像何时粘贴到可满足内容的字段:

document.onpaste = function(event){
    var items=(event.clipboardData || event.originalEvent.clipboardData || event.dataTransfer).items;
    for(i in items){
        var item=items[i];
        if(item.kind==='file' && ~item.type.indexOf("image")){ //is file and is image
            event.preventDefault();
            var blob=item.getAsFile();
            console.log(item, blob);
            var data=new FormData();
            data.append('fname','imageFromClipboard'); //extension for file name??
            data.append('newImage',blob);
            sendImage(data); //just a $.POST request
        }
    }
}

和接收图片POST请求的PHP代码:

$image = $_FILES['newImage'];
$imageFileType = pathinfo($image["name"],PATHINFO_EXTENSION); // obviously doesn't work
$imageId = new ImageId();
$imageName = $imageId.".".$imageFileType;
// [... some checks on the file ...]
$targetFile = "../../images/uploads/{$imageName}";
if($imageOK && move_uploaded_file($image["tmp_name"],$targetFile)){ 
//[ do stuff ]

如何从blob中可靠地获得文件扩展名?我应该用JS还是PHP来做?对于所有可能的图像文件类型是否可以在斜杠处分割MIME类型(例如"image/png")并将其用作扩展名?