File Download - Not able to click on Open/Save

The Script below does not click on Open button or Save button, but clicks on Cancel button without any probelm.

Autoit Info:
[CLASS:Button; TEXT:Open; INSTANCE:1] -> Open button
[CLASS:Button; TEXT:Open; INSTANCE:2] -> Save button
[CLASS:Button; TEXT:Open; INSTANCE:3] -> Cancel button

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The focus moves to the Open & Save buttons. But Click does not happen. Click on Cancel button happens without any probelm.Please advise.
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Script:
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If WinExists("File Download","Do you want to open or save this file?") Then
   MsgBox(0,"","Window Exists")
   WinActivate("File Download","Do you want to open or save this file?")
   ControlClick("File Download","Do you want to open or save this file?","[CLASS:Button; TEXT:Open; INSTANCE:1]")
   Send("{ENTER}")
Else
   MsgBox(0,"","Window Not Exists")
EndIf