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
