How to Restrict File Types in File Upload Control using ASP.NET

Hi,
 
I have a File Upload Control in a form and it allows only selected file types.
I implemented this on the button_click event of the page.
 
In web config:

<appSettings>
    <add key="FileSize" value="2097152"/>
    <add key="FileType" value=".txt,.xlsx,.xls,.docx,.doc,.ppt,.pptx,.pdf"/>
  </appSettings>

 
In btnSubmit_Click():
 

string[] allowExtentions = (ConfigurationManager.AppSettings["FileType"]).Split(',');
if (allowExtentions.Contains(Path.GetExtension(FileUp.PostedFile.FileName)) || Path.GetExtension(FileUp.PostedFile.FileName) == "")
{
    //custom code
}

 
Is there any other way to implement the file type restrictions of File Upload Control ?? Please Help
Cos, I want it to check before postback of the page.
In this implementation, it accepts the file first and later throws an error message on a label after the post back which I don't want.
 
Waiting for your valuable comments.
 
Thanks,
Phanindra Bitla