I am testing opening multiples "Save as" dialog browser windows to make sure each dialog will download the correct file. As a workaround I am sending user's keyboard keys to control a dialog. But it only works for one dialog with the focused window.
IWebElement element = driver.FindElement(By.Id("uploadhere"));
element.SendKeys("C:\\Some_Folder\\MyFile.txt");
Is there a way to switch or select a specific "Save as" dialog?