how to validate images from customer control?

public void user(object sender, ServerValidateEventArgs args)
{
if (Path.GetExtension(pp.PostedFile.FileName) == ".png" || Path.GetExtension(pp.PostedFile.FileName).ToUpper() == ".gif".ToUpper() || Path.GetExtension(pp.PostedFile.FileName).ToUpper() == ".jpg".ToUpper())
{
args.IsValid = true;
}
else
{
args.IsValid = false;
}
}


<input type=file id="pp" runat="server" />

<asp:CustomValidator ID="CustomValidator1"

ControlToValidate="pp"

OnServerValidate="user" ValidationGroup="l"

Text="only Images Format!"

runat="server"/>

<asp:Button ID="btm" runat="server" ValidationGroup="l" Text="submit"

onclick="btm_Click" />