Cannot add custom data attributes to document field

[mattr]

The custom govuk_document_field defines a stimulus controller and actions to provide additional functionality over the default file field behaviour; however, we cannot (easily) amend this in a given form to include additional stimulus behaviour.

Use case

I want to use javascript to validate the size of a file before it is uploaded.

We can work around this limitation by inheriting from the original document field controller:

import KoiDocumentFieldController from "koi/controllers/document_field_controller";

export default class DocumentFieldController extends KoiDocumentFieldController {
  onUpload(e) {
    super.onUpload(e);
    // custom code goes here
  }
}

This only allows us to hook into existing actions, and not to define any custom behaviours, however.

Alternatively, we can use the base govuk_file_field; however, the trade-off is that we lose the additional functionality that Katalyst has invested in the file upload UX.