清空input file 的值

// 清空file input
var Upload = {
    clear: function(id){
        var up = (typeof id=="string")?document.getElementById(id):id;
        if (typeof up != "object") return null;
        var tt = document.createElement("span");
        tt.id = "__tt__";
        up.parentNode.insertBefore(tt,up);
        var tf = document.createElement("form");
        tf.appendChild(up);
        document.getElementsByTagName("body")[0].appendChild(tf);
        tf.reset();
        tt.parentNode.insertBefore(up,tt);
        tt.parentNode.removeChild(tt);
        tt = null;
        tf.parentNode.removeChild(tf);
    },
    clearForm: function(){
        var inputs,frm;
        if (arguments.length == 0)
        {
            inputs = document.getElementsByTagName("input");
        }else{
            frm = (typeof arguments[0] == "string")?document.getElementById(arguments[0]):arguments[0];
            if (typeof frm != "object") return null;
            inputs = frm.getElementsByTagName("input");
        }
        var fs=[];
        for ( var i=0; i<inputs.length; i++ )
        {
            if (inputs[i].type == "file") fs[fs.length] = inputs[i];
        }
        var tf = document.createElement("form");
        for ( var i=0; i<fs.length; i++ )
        {
            var tt = document.createElement("span");
            tt.id = "__tt__" + i;
            fs[i].parentNode.insertBefore(tt, fs[i]);
            tf.appendChild(fs[i]);
        }
        document.getElementsByTagName("body")[0].appendChild(tf);
        tf.reset();
        for ( var i=0; i<fs.length; i++)
        {
            var tt = document.getElementById("__tt__" + i);
            tt.parentNode.insertBefore(fs[i],tt);
            tt.parentNode.removeChild(tt);
        }
        tf.parentNode.removeChild(tf);
    }
}