Multiple File Inputs / When Post Back Request Files Collection Is Always Empty?

I am using Asp.net and requirement specifies i use html input to post files to server rather than asp:fileupload.[ Reason : Add more html input file controls similar to CPanel file manager.(i.e) clicking on link adds another file input from which user can select another file*.But when ever i post back Request Files collection is always empty.*
HTML FORM:

<form method="post" enctype="multipart/form-data" action="documents.aspx">
<div>
<input type="file" name="attachment" id="attachment" onchange="validate(this)" />
<span class="none">Filename should be within (1-25) letters long. Can Contain only letters
& numbers</span>
<div id="moreUploads">
</div>
<div id="moreUploadsLink" style="display: none;">
<a href="javascript:addFileInput();">Attach another File</a></div>
<input type="submit" id="btnSubmit" />
</div>
</form>

Javascript:

var upload_number = 2;function addFileInput() {
try {
var fileUpload = document.getElementById("attachment");
var elemSpan = nextElement(fileUpload).cloneNode(true);
var elemDiv = document.getElementById("moreUploads");
var d = document.createElement("div");
var file = document.createElement("input");
file.setAttribute("type", "file");
file.setAttribute("id", "attachment" + upload_number);
file.setAttribute("onchange", "validate(this)");
d.appendChild(file);
d.appendChild(elemSpan);
elemDiv.appendChild(d);
upload_number++;
} catch (err) { alert(err);}}

the validate(this) is a function that validates file types on client.When validation succeeds then the link to add more file inputs is enabled. Could someone throw somelight on this.