<form id="uploadfile" name="uploadfile" class="" target="upload" action="http://web.qq.com/cgi-bin/cface_upload" method="POST" enctype="multipart/form-data">
    <input id="custom_face" name="custom_face" type="file">
    <input name="f" type="hidden" value="fallback">
    <input name="go" type="button" value="Upload File" onclick="Upload();">
</form>

  javascript代码片段：

  function Upload()
{
    var form=document.getElementById("uploadfile");
    var face=document.getElementById("custom_face");
    //ReadFFFile(face);
    var fileSize=GetFileSize(face);
    if(fileSize>1024*1024)
    {
        alert("文件不能大于1M");
    }
    else
    {
        form.submit();
    };
};
function GetFileSize(fileObj)　　
{　　
　  var image=new Image();
    var filePath=fileObj.value;
　  image.dynsrc=filePath;
　  var fileSize=image.fileSize||0;
    if(fileSize==0)
    {
        fileSize=fileObj.files[0].fileSize;
    };
    return fileSize;
};