XML Sitemap - broken URL's when including files

Problem/Motivation

Adding 'File' as a custom entity through the XML Sitemap configuration results in broken sitemap URL's.

Steps to reproduce

1. Configure Custom Entity Settings at: admin/config/search/xmlsitemap/entities/settings
2. Tick file, configure Sitemap Settings to be included, save changes
3. Rebuild sitemap links: admin/config/search/xmlsitemap/rebuild
4. Rebuild cron: admin/config/system/cron
5. Navigate to YOUR_DOMAIN/sitemap.xml
6. File links are broken

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes