Multiple Upload with preview

<p id="upload-area">
   <input id="File1" type="file" runat="server" size="60" onchange="preview(this)"/>
   <br>
<img alt="" id="previewField" src="spacer.gif">
<br>
</p>
 
<input id="AddFile" type="button" value="Add file" onclick="addFileUploadBox()" />
<p><asp:Button ID="btnSubmit" runat="server" Text="Upload Now"  OnClick="btnSubmit_Click" /></p>
</div>
<script type="text/javascript">
function addFileUploadBox()
{
    if (!document.getElementById || !document.createElement)
        return false;
		
    var uploadArea = document.getElementById ("upload-area");
	
    if (!uploadArea)
        return;
 
    var newLine = document.createElement ("br");
    uploadArea.appendChild (newLine);
	
    var newUploadBox = document.createElement ("input");
	
    // Set up the new input for file uploads
    newUploadBox.type = "file";
    newUploadBox.size = "60";
	
    // The new box needs a name and an ID
    if (!addFileUploadBox.lastAssignedId)
        addFileUploadBox.lastAssignedId = 100;
	    
    newUploadBox.setAttribute ("id", "dynamic" + addFileUploadBox.lastAssignedId);
    newUploadBox.setAttribute ("name", "dynamic:" + addFileUploadBox.lastAssignedId);
    uploadArea.appendChild (newUploadBox);
    addFileUploadBox.lastAssignedId++;
}
 
 
</script>