上传图片，实时预览

<input type="file" accept="image/*" class="add_btn">
<div class="upload"></div>

$(".add_btn").change(function(){
var files = this.files; //获取路径
var img = new Image();
var reader = new FileReader();//读取文件
reader.readAsDataURL(files[0]);
reader.onload = function(e){
var mb = (e.total/1024)/1024;
if(mb>= 2){
alert('文件大小大于2M');
return;
}
img.src = this.result;
$(".upload").innerHTML = '';
$(".upload").append(img);
}
});