Essay questions: no message given if you don't submit enough files.

Steps to reproduce:

1. Create an essay question which requires the students to upload two files.
2. Preview the question.
3. Add one file to your response, and click the 'Save' button.

Expected result: the question state is shown as 'Incomplete answer' and there is some text (usually styled red) which says something like "You must submit at least two files."

Acutal result: The question state is 'Incomplete answer', but no explanation is given as to why.