file upload issue when user clcik multiple times

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C#

ASP.NET

i need a solution that when user click multiple times on file upload control then old selected file should not remover or update latest , suppose when user select 5 file at a time and again he need 2 more file to add then it will become 7 file selected in control not 2.
my control code contain
aspx

<pre lang="HTML">
script src="http://ajax.googleapis.com/ajax/libs/jquery/2.0.2/jquery.min.js" type="text/javascript"></script>
<script type="text/javascript">
    function dis() {
        $(document).ready(function () {


            $(".addm").on("click", function () {

                $(".files").click();
            });
            $(".sss").on("click", function () {

                $(".files").click();
            });

            function readImage(file) {

                var reader = new FileReader();
                var image = new Image();

                reader.readAsDataURL(file);
                reader.onload = function (_file) {
                    image.src = _file.target.result;              // url.createObjectURL(file);
                    image.onload = function () {
                        var w = this.width,
                h = this.height,
                t = file.type,                           // ext only: // file.type.split('/')[1],
                n = file.name,
                s = ~ ~(file.size / 1024) + 'KB';
                        $(".f").css("visibility", "visible");
                        $('.f .g').append('<img style="max-width:102px; max-height:102px; margin-left:3px;" src="' + this.src + '"> ' + '   ');
                        $('.ms').css("display", "none");
                    };
                    image.onerror = function () {
                        alert('Invalid file type: ' + file.type);
                    };
                };

            }
            $(".files").change(function (e) {
                if (this.disabled) return alert('File upload not supported!');
                var F = this.files;
                if (F && F[0]) for (var i = 0; i < F.length; i++) readImage(F[i]);
            });
        });
    }

</script>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <script type="text/javascript">
            Sys.Application.add_load(dis);
        </script>
    </ContentTemplate>
</asp:UpdatePanel>
<div class="comment-box">
    <ul>
        <li><span>
            <img src="../../images/status_icon.png" alt="" /></span><asp:LinkButton ID="lnkUpdateStatus"
                runat="server" OnClick="lnkUpdateStatus_Click">Update Status</asp:LinkButton>
        </li>
        <li><span>
            <img src="../../images/add-photo-icon.png" alt="" /></span>
            <asp:LinkButton ID="lnkAddPhotoVideo" runat="server" OnClick="lnkAddPhotoVideo_Click">Add Photos/Video</asp:LinkButton>
        </li>
    </ul>
    <div class="text-area">
        <div class="corner">
        </div>
        <div class="corner-1">
        </div>
        <div class="clear">
        </div>
        <div class="text-part tt" id="Status"  runat="server">
            <asp:TextBox ID="txtstatus" CssClass="style h" runat="server" TextMode="MultiLine"></asp:TextBox>
            <cc1:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender3"  runat="server" TargetControlID="txtstatus"
                WatermarkText="Enter Your Status">
            </cc1:TextBoxWatermarkExtender>
            <div class="clear">
            </div>
            <div class="button-area">
                <asp:Button ID="btnsubmitstatus" runat="server" Text="Post" CssClass="submit-style g"
                    OnClick="btnsubmitstatus_Click" /></div>
        </div>
        <div class="text-part ms" id="AddPhotoAlbum"  runat="server" visible="false">
            <ul>
                <li>
                    <div class="sss">
                        <br />
                        <br />
                        "FileUpload1" class="multi files">Create Photo Album<br />
                    <br />
                    <br />
                    <br />
                </li>
            </ul>
            <div class="clear">
            </div>
            <div class="button-area">
                <%-- <asp:Button ID="btnPost" runat="server" Text="Post" CssClass="submit-style" />--%>
            </div>
        </div>
        <div class="text-part-1 f" id="AddPhotos" style="visibility: hidden">
            <asp:TextBox ID="txtPhoto" CssClass="style h" runat="server" TextMode="MultiLine"></asp:TextBox>
            <cc1:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender1"  runat="server" TargetControlID="txtPhoto"
                WatermarkText="Enter Your Status">
            </cc1:TextBoxWatermarkExtender>
            <div class="upload-image g" id="Photo">
            </div>
            <div class="addm" style="float: left;">
           <%--     ^__b style="removed: pointer;">Add More--%>
            
            </div>
            <div class="clear">
            </div>
            <div class="button-area">
                <asp:Button ID="btnAddPhoto" runat="server" Text="Post" CssClass="submit-style" OnClick="btnAddPhoto_Click" />
            </div>
        </div>
    </div>
</div>

cs

protected void btnsubmitstatus_Click(object sender, EventArgs e)
   {
       if (txtstatus.Text != "")
       {
           string User_Id = ab.GetUser_Id(uid);
           string TypeId = ab.GetTypeID("Status-Text");

           ab.InsertSlamBookStatus(txtstatus.Text, null, null, null, null, null, null, User_Id, TypeId, null, DateTime.Now.ToString(), "", "", "");
           txtstatus.Text = "";

       }
   }

   protected void lnkUpdateStatus_Click(object sender, EventArgs e)
   {
       Status.Visible = true;
       AddPhotoAlbum.Visible = false;
       //AddPhotos.Visible = false;

   }
   protected void lnkAddPhotoVideo_Click(object sender, EventArgs e)
   {
       AddPhotoAlbum.Visible = true;
       //AddPhotos.Visible = false;
       Status.Visible = false;

   }

   protected void btnAddPhoto_Click(object sender, EventArgs e)
   {
       string pt = null;
       string cpath = null;
       string PhotoFolder = "~/Photos/";
       string PhotoThumbFolder = "~/Photos/Thumb/";
       if (FileUpload1.HasFile)
       {
           string UID = ab.GetUser_Id(uid);
           string imgName = Path.GetFileName(FileUpload1.FileName);
           string date = DateTime.Now.ToString();

           while (File.Exists(Server.MapPath(PhotoFolder + imgName)))
           {
               cpath = Server.MapPath(PhotoFolder + imgName);
               pt = Server.MapPath(PhotoThumbFolder + imgName);
               File.Delete(cpath);
               File.Delete(pt);
           }

           HttpFileCollection hfc = HttpContext.Current.Request.Files;
           for (int i = 0; i < hfc.Count; i++)
           {

               HttpPostedFile hpf = hfc[i];
               if (hpf.ContentLength > 0)
               {
                   //Original Image Save
                   hpf.SaveAs(Server.MapPath(PhotoFolder) + Path.GetFileName(hpf.FileName));
                   // Load image.
                   System.Drawing.Image image = System.Drawing.Image.FromFile(Server.MapPath(PhotoFolder) + hpf.FileName);
                   // Compute thumbnail size.
                   Size thumbnailSize = GetThumbnailSize(image);
                   // Get thumbnail.
                   System.Drawing.Image thumbnail = image.GetThumbnailImage(thumbnailSize.Width, thumbnailSize.Height, null, IntPtr.Zero);
                   // Save thumbnail.
                   thumbnail.Save(Server.MapPath(PhotoThumbFolder + hpf.FileName));
                   //////////// Get Photo Type Id////////
                   string PhotoTypeId = ab.GetTypeID("Photo");
                   /////////////Insert in to Album Table////////////
                   ab.InsertSinglePhotointoAlbum(PhotoTypeId, null, date, hpf.FileName);
                   //////////////Get Album ID from Album table/////////////////////
                   string AlbumId = ab.GetAlbumID(PhotoTypeId, date, hpf.FileName);
                   /////////////Insert in to status Table////////////
                   ab.InsertSlamBookStatus(null, AlbumId, null, null, null, null, null, UID, PhotoTypeId, null, date, null, null, null);

               }
           }

           // FileUpload1.PostedFile.SaveAs(Server.MapPath(PhotoFolder) + imgName);




       }


   }
   static Size GetThumbnailSize(System.Drawing.Image original)
   {
       // Maximum size of any dimension.
       const int maxPixels = 450;
       // Width and height.
       int originalWidth = original.Width;
       int originalHeight = original.Height;
       // Compute best factor to scale entire image based on larger dimension.
       double factor;
       if (originalWidth > originalHeight)
       {
           factor = (double)maxPixels / originalWidth;
       }
       else
       {
           factor = (double)maxPixels / originalHeight;
       }
       // Return thumbnail size.
       return new Size((int)(originalWidth * factor), (int)(originalHeight * factor));
   }