JS检查上传图片的类型和大小

适用场景：检查用户上传的文件是否为图片格式且尺寸不大于2MB

$("#upload_photo").change(function(){

var file=this.files[0];//获取上传的文件

var filepath = $("#upload_photo").val();
var extStart = filepath.lastIndexOf(".");
var ext = filepath.substring(extStart, filepath.length).toUpperCase();
if (ext != ".PNG" && ext != ".JPG" && ext != ".JPEG") {
this.value ="";
alertView("头像格式须为jpg/jpeg/png","warn","","","确定");//自定义alert
changeSubmitType(-1);//修改submit的button状态为不可点击
return;
}

var fileSize = file.size;
if (fileSize > 2*1024*1024){
this.value ="";
alertView("头像大小不能超过2MB","warn","","","确定");
changeSubmitType(-1);
return;
}

changeSubmitType(1);

var reader=new FileReader();
reader.onload=function(){
// 通过 reader.result 来访问生成的 DataURL
var url=reader.result;
$("#preview").attr("src",url);
};
reader.readAsDataURL(file);
});