How to include "File upload" field in the webform composite element?
Depends on the destination folder. This webform FAQ might help you.

Edit: Webform Composite Elements do not support File elements yet. Reference on drupal here 

Follow these issues (active as of 6/20/2017) to find release updates-
https://www.drupal.org/node/2883734
, https://www.drupal.org/node/2884624