为什么我这里得到的图片大小是NaN?

Question

    <form enctype="multipart/form-data" name="form1">      <input type="file" name="file">      <input type="button" value="取得图片的大小" onClick="getSize()">    </form>    <img name="pic" alt="在这儿显示图片">function getSize(){        size = 0;        imgURL = form1.file.value;        imgURLsplice = imgURL.split("\\").pop();        imgURLsplicelocal ="../assets/img/" + imgURLsplice;        img = new Image();        img.src = imgURLsplicelocal;        pic.src = imgURLsplicelocal;        if (size <= 0){            size = img.fileSize / 1024;        }        alert("图片大小为：" + size + "KB");    }