What is the code for allowmultiple=“true”? (fileupload)

I want to upload multiple files with fileupload. That works :) But it should upload multiple files with a Limit of 5 files. 

Here is my behind code:

protected void btnUpload_Click(object sender, EventArgs e)
        {

            StartUpLoad();
        }

        private void StartUpLoad()
        {
            string imgName = FileUpload1.FileName;
            string imgPath = "~/Uploads/" + imgName;
            int imgSize = FileUpload1.PostedFile.ContentLength;
            string ext = System.IO.Path.GetExtension(this.FileUpload1.PostedFile.FileName);
            if (FileUpload1.PostedFile != null && FileUpload1.PostedFile.FileName != "")
            {
                if (ext.ToUpper().Trim() != ".JPG" && ext.ToUpper() != ".PNG" && ext.ToUpper() != ".GIF" && ext.ToUpper() != ".JPEG")
                {
                    Page.ClientScript.RegisterClientScriptBlock(typeof(Page), "Alert", "alert('Only .jpg, .png oder .gif!')", true);
                }

                else
                {

                    FileUpload1.SaveAs(Server.MapPath("~/Uploads/") + FileUpload1.FileName);
                    Page.ClientScript.RegisterClientScriptBlock(typeof(Page), "Alert", "alert('Saved!')", true);

                }
            }
        }