Hi simflex,

Refer below sample.

HTML

<script type="text/javascript" src="scripts/jquery-1.3.2.min.js"></script>
<script type="text/javascript" src="scripts/jquery.uploadify.js"></script>
<script type="text/javascript">
    $(function () {
        $("[id*=FileUpload1]").fileUpload({
            'uploader': 'scripts/uploader.swf',
            'cancelImg': 'images/cancel.png',
            'buttonText': 'Attach Files',
            'script': 'UploadCS.ashx',
            'folder': 'uploads',
            'multi': true,
            'auto': true,
            'scriptData': { key: '<%=Key %>' },
            'onSelect': function (event, ID, file) {
                $("#attachedfiles tr").each(function () {
                    if ($("td", this).eq(0).html() == file.name) {
                        alert(file.name + " already uploaded.");
                        $("[id*=FileUpload1]").fileUploadCancel(ID);
                        return;
                    }
                });
            },
            'onComplete': function (event, ID, file, response, data) {
                $("#attachedfiles").append("<tr><td>" + file.name + "</td><td><a href = 'javascript:;'>[x]</a></td></tr>");
            }
        });
    });

    $("#attachedfiles a").live("click", function () {
        var row = $(this).closest("tr");
        var fileName = $("td", row).eq(0).html();
        $.ajax({
            type: "POST",
            url: "CS.aspx/RemoveFile",
            data: '{fileName: "' + fileName + '", key: "<%=Key %>" }',
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function () { },
            failure: function (response) {
                alert(response.d);
            }
        });
        row.remove();
    });
</script>
<table>
    <tr>
        <td>
            To:
        </td>
        <td>
            <asp:TextBox ID="txtTo" runat="server"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            Subject:
        </td>
        <td>
            <asp:TextBox ID="txtSubject" runat="server"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            Body:
        </td>
        <td>
            <asp:TextBox ID="txtBody" runat="server" TextMode="MultiLine"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
        </td>
        <td>
            <asp:FileUpload ID="FileUpload1" runat="server" />
        </td>
    </tr>
    <tr>
        <td>
        </td>
        <td>
            <table id="attachedfiles">
            </table>
        </td>
    </tr>
    <tr>
        <td>
        </td>
        <td>
            <asp:Button ID="btnSend" runat="server" Text="Send" OnClick="btnSend_Click" />
        </td>
    </tr>
</table>

Namespaces

C#

using System.Web.Services;
using System.Net.Mail;
using System.IO;

Code

UploadCS.ashx

<%@ WebHandler Language="C#" Class="UploadCS" %>
using System;
using System.Web;
using System.IO;
using System.Collections.Generic;
public class UploadCS : IHttpHandler {
    public void ProcessRequest (HttpContext context) {
        context.Response.ContentType = "text/plain";
        context.Response.Expires = -1;
        try
        {
            List<HttpPostedFile> files = (List<HttpPostedFile>)context.Cache[context.Request.QueryString["key"]];
            HttpPostedFile postedFile = context.Request.Files["Filedata"];
            files.Add(postedFile);
            string filename = postedFile.FileName;
            postedFile.SaveAs(HttpContext.Current.Server.MapPath("~/Uploads/") + filename);
            context.Response.Write(filename);
            context.Response.StatusCode = 200;
        }
        catch (Exception ex)
        {
            context.Response.Write("Error: " + ex.Message);
        }
    }
 
    public bool IsReusable {
        get {
            return false;
        }
    }
}

CS.aspx.cs

protected string Key
{
    get { return ViewState["Key"].ToString(); }
    set { ViewState["Key"] = value; }
}
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        this.Key = Guid.NewGuid().ToString();
        Cache[this.Key] = new List<HttpPostedFile>();
    }
}

[WebMethod]
public static void RemoveFile(string fileName, string key)
{
    List<HttpPostedFile> files = (List<HttpPostedFile>)HttpContext.Current.Cache[key];
    File.Delete(HttpContext.Current.Server.MapPath("~/Uploads/") + fileName.ToLower());
}