Asp fileupload need to allow only PDF file from clent side

<asp:asyncfileupload onuploadedcomplete="FileUploadComplete" xmlns:asp="#unknown">
                                    runat="server" ID="AsyncFileUploadImage" Width="400px" UploaderStyle="Modern"
                                    CompleteBackColor="White" UploadingBackColor="#0A7AC5" ThrobberID="imgloading"
                                    OnClientUploadError="uploadError" CssClass="fileupload" TabIndex="6" OnClientUploadStarted="uploadImageStart" /></asp:asyncfileupload>

function uploadImageStart(sender, args) {
    var filename = args.get_fileName();

    var filext = filename.substring(filename.lastIndexOf(".") + 1);
    filext = filext.toLowerCase();
    if (filext == "pdf") {
        return true;
    }
    else {
        var err = new Error();
        err.name = 'Fill Ext Error';
        err.message = 'Not valid file. Only .pdf files allowed';
        throw (err);
        return false;
    }
}

function uploadError(sender, args) {
    alert(args.get_errorMessage());

}

<script type="text/javascript">
    var validFilesTypes=["bmp","gif","png","jpg","jpeg","doc","xls"];
    function ValidateFile()
    {
      var file = document.getElementById("<%=FileUpload1.ClientID%>");
      var label = document.getElementById("<%=Label1.ClientID%>");
      var path = file.value;
      var ext=path.substring(path.lastIndexOf(".")+1,path.length).toLowerCase();
      var isValidFile = false;
      for (var i=0; i<validfilestypes.length;>      {
        if (ext==validFilesTypes[i])
        {
            isValidFile=true;
            break;
        }
      }
      if (!isValidFile)
      {
        label.style.color="red";
        label.innerHTML="Invalid File. Please upload a File with" +
         " extension:\n\n"+validFilesTypes.join(", ");
      }
      return isValidFile;
     }
</script></script>

protected void btnUpload_Click(object sender, EventArgs e)
{
    string[] validFileTypes={"bmp","gif","png","jpg","jpeg","doc","xls"};
    string ext = System.IO.Path.GetExtension(FileUpload1.PostedFile.FileName);
    bool isValidFile = false;
    for (int i = 0; i < validFileTypes.Length; i++)
    {
        if (ext == "." + validFileTypes[i] )
        {
            isValidFile = true;
            break;
        }
    }
    if (!isValidFile)
    {
        Label1.ForeColor = System.Drawing.Color.Red;
        Label1.Text = "Invalid File. Please upload a File with extension " +
                       string.Join(",", validFileTypes);
    }
    else
    {
        Label1.ForeColor = System.Drawing.Color.Green;
        Label1.Text = "File uploaded successfully.";
    }
}