deleteOwnFile method
deleteFile: deletes an own file that is not attached; 409 conflict for an attached
one.
Implementation
void deleteOwnFile(String principalId, String fileId) {
final f = _ownFile(principalId, fileId);
if (f.attachedTo != null) throw const FakeProblem(409, 'conflict');
files.remove(fileId);
}