Whenever the user interacts with an <input type='file' /> HTML element, WebKit will need to show a dialog to choose one or more
files to be uploaded to the server along with the rest of the form data. For that to happen in a general way, instead of just opening a
FileChooserDialog (which might be not desirable in some cases, which could prefer to use their own file chooser
dialog), WebKit will fire the run_file_chooser signal with a
FileChooserRequest object, which will allow the client application to specify the files to be selected, to
inspect the details of the request (e.g. if multiple selection should be allowed) and to cancel the request, in case nothing was selected.
In case the client application does not wish to handle this signal, WebKit will provide a default handler which will asynchronously run a
regular FileChooserDialog for the user to interact with.