C# - Javascript Function Don't Run At Once?

I've some function javascript to check loaded file with FileUpload Control

<script language="javascript" type="text/javascript">
function CheckFileBeforeUpdate()
{
var filePath = document.getElementById('<%= this.upFile.ClientID %>').value;
var validExtension = 'xml';
var ext = filePath.substring(filePath.lastIndexOf('.') + 1).toLowerCase();
if (ext == validExtension) return true;
alert('The file extension ' + ext.toUpperCase() + ' is not allowed!');
return false;
}
</script>

and it's called with Button: OnClientClick="return CheckFileBeforeUpdate();

<asp:Button ID="LoginButton" runat="server" CommandName="Login" Text="Log In"
onclick="LoginButton_Click" OnClientClick="return CheckFileBeforeUpdate();" />

So there's unusual situation: I choose appropiate file, click Login...and it goes on to next functions despite inadequate extension.

Otherwise if I e.g. click FileUpload, but select no file.
Next time I choose some file (even with bad extension) and then function run (shows alert).

Why there's sth like blockade? What can I do to change CheckFileBeforeUpdate() runs everytime?

EDIT
if I select no file there's show alert("Select file to log in"); of course. And then is lock release

[Code]....