Currently no nice way to extend the default file extensions. A public array exists in the EditableFileField but never is applied to the form field.
Allowed extensions should probably be in the cms as a comma separated list (with the standard defaults).
Wilr, what is currently best way to set or limit allowed file extensions for a UserDefinedForm EditableFileField in SS3.1?
It uses the default File.allowed_extensions config value. You can only change this site wide.