For your requirement, I have created a demo using Modal Popup
Have a look below
$(function () {
modalPosition();
$(window).resize(function () {
modalPosition();
});
$('.openModal').click(function (e) {
$('.modal, .modal-backdrop').fadeIn('fast');
e.preventDefault();
});
$('.close-modal').click(function (e) {
$('.modal, .modal-backdrop').fadeOut('fast');
});
});
function modalPosition() {
var width = $('.modal').width();
var pageWidth = $(window).width();
var x = (pageWidth / 2) - (width / 2);
$('.modal').css({ left: x + "px" });
}
.modal-backdrop
{
background-color: rgba(0, 0, 0, 0.61);
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
display: none;
}
.modal
{
width: 500px;
position: absolute;
top: 25%;
z-index: 1020;
background-color: #FFF;
border-radius: 6px;
display: none;
}
.modal-header
{
background-color: #333;
color: #FFF;
border-top-right-radius: 5px;
border-top-left-radius: 5px;
}
.modal-header h3
{
margin: 0;
padding: 0 10px 0 10px;
line-height: 40px;
}
.modal-header h3 .close-modal
{
float: right;
text-decoration: none;
color: #FFF;
}
.modal-footer
{
background-color: #F1F1F1;
padding: 0 10px 0 10px;
line-height: 40px;
text-align: right;
border-bottom-right-radius: 5px;
border-bottom-left-radius: 5px;
border-top: solid 1px #CCC;
}
.modal-body
{
padding: 0 10px 0 10px;
}
<asp:Button ID="btn_open" runat="server" Text="Open Modal" CssClass="openModal" />
<div class="modal">
<div class="modal-header">
<h3>
File Upload <a class="close-modal" href="#">×</a></h3>
</div>
<div class="modal-body">
<p>
<asp:FileUpload ID="FileUpload1" runat="server" AllowMultiple="true" />
</div>
<div class="modal-footer">
<asp:Button ID="btn_close" runat="server" Text="Upload Document" CssClass="modalOK close-modal" />
</div>
</div>
<div class="modal-backdrop">
</div>
Dont forget to add the Jquery Plugin
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
Let me know if you have any doubts.
Hope that helps you
Have a look at the below screenshot
