First you have to download JavaScript and CSS file from this URL 

https://github.com/JeremyFagis/dropify


Now add Latest JQuery in your program from below URL

https://developers.google.com/speed/libraries#jquery


after that add code in your head section in your asp.net page


<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js" type="text/javascript"></script>

    <link href="../assets/vendors/dropify/dist/dropify.min.css" rel="stylesheet" />

    <script src="../assets/vendors/dropify/dist/dropify.min.js"></script>

<script type="text/javascript">

        $(document).ready(function () {

            $('.dropify').dropify({

                messages: {

                    'default': 'Click or Drag',

                    'replace': 'Drag and drop or click to replace',

                    'remove': 'Remove',

                    'error': 'Ooops, something wrong happended.'

                }

            });

     });

 </script>


add FileUpload Control from Toolbox and set the property of CssClass = "dropify"

<asp:FileUpload ID="FileUpload1" runat="server" CssClass="dropify" data-height="150"/>


Now run your project or website.

and Enjoy.