I've figured out how to update answer.twig to customize the file upload answer page, but am curious as to whether or not that's the proper way to do such customization.  In other words, for updating themes I'd make a copy of custom.css, or one of the twig files, vs. editing directly.  

You can create a QuestionTheme

Some sample : github.com/tpartner?tab=repositories

Interesting, and I'm going to assume this is the "right" way to modify how questions are rendered. Appreciate the link.

You can do it in your survey theme too : gitlab.com/SondagesPro/SurveyThemes/skel...vey/questions/answer

But then : no update when core are updated , and no choice for your usezr.
But for upload : it can be interseting