I'm having the exact same issue. Below are my attempts to get this to work.
The above will open the File Download dialog and it will just hang until it times out.
//Enter path to file location
//This code will only open the download dialog and click Cancel
DownloadDialogsHandler handler =
DownloadDialogsHandler(Manager.ActiveBrowser, DialogButton.CANCEL, fullPath, Manager.Desktop);
I've also tried the code below
, DownloadOption.Cancel, fullPath, 10000);
The code above won't open the File Download dialog. The overall result is a success, but I don't believe the dialog is being called at all.
I've also trying the built in dialog handlers in the development edition of WebAii, but they react the same way the first piece of code does.