js限制上传文件大小

 form表达提交文件，如果文本过大，就会报413 Request Entity Too Large(请求实体太大)；所以要在页面就限制文本大小。

HTML :

<form id = "adressForm" method="post" enctype="multipart/form-data">
             <li class="ovh">
                <div class="updatas fl">
                    <p class="p01 ovh">
                    <span class="ups db rdu5" id="PositiveImg"><input id="upPositive" class="filePrew" type="file" size="3" name="cidFront" onchange="fileChange(this)" /></span>
                    </p>
                </div>
                <div class="updatas fl">
                    <p class="p02 ovh">
                    <span class="ups db rdu5" id="NegativeImg"><input id="upNegative" class="filePrew" type="file" size="3" name="cidBack" onchange="fileChange(this)" /></span>
                    </p>
                </div>
             </li>
         </ul>
      </form>

JS：

function fileChange(target) {
    var fileSize = 0;         
    if ((navigator.userAgent.indexOf('MSIE') >= 0) && (navigator.userAgent.indexOf('Opera') < 0) && !target.files) {//如果是ie   
        alert("is IE");
        var filePath = target.value;     
        var fileSystem = new ActiveXObject("Scripting.FileSystemObject");        
        var file = fileSystem.GetFile (filePath);     
        fileSize = file.Size;   
    } else {    
        fileSize = target.files[0].size;     
    }   
    var size = fileSize / 1024;   
    if(size > 5000){  
        dialogMsg("附件不能大于5M");
        target.value="";
        return
    }
    var name=target.value;
    var fileName = name.substring(name.lastIndexOf(".")+1).toLowerCase();
    var imageArray = ["gif","jpg","jpeg","png","bmp"];
    if(fileName == null || fileName == "" || imageArray.indexOf(fileName) < 0){
        dialogMsg("格式不对");
        target.value="";
        return
    }
}