用JS怎么判断上传文件是否为空

    <form name="form1" action="uploadPosdetailFile.html" method="post" ENCTYPE="multipart/form-data">
    <div class="form" >
            <p>
                <span class="req"><input id="startDateTxt"  name="startDateTxt" class="field size4" title="Enter the date" /></span>
                <label>Start Date: <span>(Date format:MM/dd/yyyy,eg:01/01/2014)</span></label>
            </p>
            
            <p>
                <span class="req"><input id="endDateTxt"  name="endDateTxt" class="field size4" title="Enter the date" /></span>
                <label>End Date: <span>(Date format:MM/dd/yyyy,eg:12/25/2014)</span></label>
            </p>
            
            <p>
                <span class="req"><input type="file"  name="uploadFileCtrl" class="field size4" title="Choose the file" /></span>
                <label>Upload file: <span>(click browse to choose)</span></label>
            </p>
    </div>
    
    <div class="buttons">
        <input id="queryBtn" type="button" class="button" value="Submit" />
    </div>
    
    </form>

    $("#queryBtn").click(
        function(){
            ......
            
            var s=document.form1.uploadFileCtrl.value; 
            if(s==""){
                alert("Please choose a posdetail file.");
                document.form1.uploadFileCtrl.focus();
                return;
            }
            
            showLoadingWnd(‘Uploading,please wait...‘);
            document.form1.submit();
        }
    );