file控件只有一个可以的写数据地方，但可读，其它控件一般是可读也可写

<script>

function canRead()

{

alert(document.getElementById("upload").value);

}

function canWrite()

{

document.getElementById("upload").value="c://1.jpg";

alert(document.getElementById("upload").value);

}

</script>

<form id="form1" name="form1" enctype="multipart/form-data" method="post" action="">

<input type="file" name="upload" id="upload"/>

<br>

<input type="button" value="可写" onclick="canRead()">

<input type="button" value="可读" onclick="canWrite()">

</form>