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

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ASP.NET

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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