When submitting files through the form template upload field, we were having an issue where, although a validation error message was being presented to the user, the upload was still coming through, because `PickFilesButton`'s validation happens **after** the Uppy mixin finished the upload and hit `uploadDone`.
This PR adds a new overridable method to the Uppy mixin and overrides it with the custom validation, which now happens before the file is sent.
Additionally, we're now also using `uploadingOrProcessing` as the source of truth to show the upload/uploading label, which seems more reliable.
2201f8f7dc