What are the "Files" that Metatag module allows adding Metadata to?
They are all the files managed by Drupal through attachments, images &amp; file fields. In D7, these managed files are entities. As such, the metatag module supports it. 

This isn't useful with the metatag module alone, but if you had a configuration where files had their own display page (e.g. the file entity &amp; media module), it could be utilized.