uploadFile abstract method
POST /{resource}/upload — uploads a file for a single-file upload
field. field is the field's statePath, so a nested field is
addressable.
Fails with UploadFailed rather than throwing, including when the
host's transport has not implemented the optional
FilamentUploadTransport port — the same never-throw contract every
other write has.
Implementation
Future<UploadResult> uploadFile(
String resourceKey,
String field, {
required List<int> bytes,
required String filename,
});