Adds a custom field to the collection.
$au.uploader('uploaderID').metadata().addCustomField(name, value, add);
The custom field name.
The value to add.
The value specifies how to handle a situation when a field with a given name is already added to the POST requst. If
true, the previously added value
will be preserved and a new field with the same name will be added. Otherwise, this value will be overwritten.