how to do client side validation for AsyncFileUpload in

I tried the validator with file upload and it works fine but for AsyncFileUpload i get a error

Control 'AsyncFileUpload1' referenced by the ControlToValidate 
         property of 'RegularExpressionValidator1' cannot be validated.

is there any method by which I can do validation before uploading the file. Thanks

   <asp:AsyncFileUpload ID="AsyncFileUpload1" runat="server" UploadingBackColor="Yellow"
        OnUploadedComplete="ProcessUpload" OnClientUploadComplete="showUploadConfirmation"
        ThrobberID="spanUploading" />
     <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ErrorMessage="Only zip file is allowed!"
        ValidationExpression="^.+(.zip|.ZIP)$" ControlToValidate="AsyncFileUpload1"> </asp:RegularExpressionValidator>


I doubt whether with Async File Upload you can do this, as it will check at OnUploadedComplete at server side and OnClientUploadComplete at client side.

So my conclusion is that it may not be possible.

check this link for more info

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