<asp:FileUpload ID="FileUpload1" runat="server"
onchange="change(this);" />


//js
function change(fileupload) {

            if (fileupload.value.indexOf('jpg') == '-1') {
                document.getElementById('TextBox1').value =
'Nopicture';
                alert('Select *.jpg file only');
            }
            if (fileupload.value.indexOf('jpg') != '-1') {
                document.getElementById('TextBox1').value =
'haspicture';
             }

         }

         function changevalue() {
             if
(document.getElementById('FileUpload1').value == '') {
                 alert('Please select a file');
                 return false;
             }
              if
(document.getElementById('FileUpload1').value !='')
               {
                  if
(document.getElementById('FileUpload1').value.indexOf('jpg')
== '-1') {

                      alert('upload *.jpg images only');
                      return false;
                  }
              }

 
//code behind
protected void btnUpload_Click(object sender, EventArgs e)
    {
        if (btnUpload.Text == "Upload")
        {
            if (FileUpload1.HasFile)
            {

                Stream imgStream =
FileUpload1.PostedFile.InputStream;

                int imgLen =
FileUpload1.PostedFile.ContentLength;
                byte[] img = new byte[imgLen];
                int n = imgStream.Read(img, 0, imgLen);
                Session["Picture"] = img;
                lblMessage.Text = "Attached";
                btnUpload.Text = "Remove";
            }
            else
            {
                Session["Picture"] = null;
            }

        }
        else
        {
            Session["Picture"] = null;
            btnUpload.Text = "Upload";
            lblMessage.Text = "Removed";
        }
    }