While testing the issue MDL-55289 I realized that attaching files to
workshop example submissions does not work at all and throws an error.
The reason was that in MDL-50794 (996f7e82), the variables $contentopts
and $attachmentopts were replaced with the result of the methods
submission_content_options() and submission_attachment_options().
But I forgot to perform the full refactoring in exsubmission.php too.
Attached behat test should cover both issues on this branch.