This event occurs when a file download is initiated either by a
user clicking on a link or navigating to a URL which specifies content
that cannot be rendered in the browser and the FileUrl argument
specifies the URL for the file being downloaded, the FileName
argument is the path to the local file that will be created. The local
file name is passed by reference to the event and can be changed in
the event to specify a different file path and/or name if needed.
To prevent the file from being downloaded, set the Cancel
argument to True inside the event handler. If the download is
canceled, the browser will not show the normal progress dialog in the
upper-right corner of the control window.