<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; i++)

      {

        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>

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.";

    }

}

Protected Sub btnUpload_Click(ByVal sender As Object, ByVal e As System.EventArgs)

 Dim validFileTypes As String() = {"bmp","gif","png","jpg", "jpeg","doc","xls"}

 Dim ext As String = System.IO.Path.GetExtension(FileUpload1.PostedFile.FileName)

 Dim isValidFile As Boolean = False

 For i As Integer = 0 To validFileTypes.Length - 1

    If ext = "." & validFileTypes(i) Then

        isValidFile = True

        Exit For

    End If

 Next

 If Not isValidFile Then

    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."

 End If

End Sub

<asp:FileUpload ID="FileUpload1" runat="server" />

<asp:Button ID="btnUpload" runat="server" Text="Upload" 

   OnClientClick = "return ValidateFile()"  OnClick="btnUpload_Click"  />

<br />

<asp:Label ID="Label1" runat="server" Text="" />