Fires when one or more files are about to be uploaded. If you cancel the event, the upload is prevented.
Arguments for the
The optional object that is sent to the
upload handler in the form of key/value pair.
The list of the files that are about to be uploaded.
The headers of the request.
If the event is prevented by any of its subscribers, returns
true if the default action was prevented. Otherwise, returns
Prevents the default action for a specified event.
In this way, the source component suppresses the built-in behavior that follows the event.