Backup and Migrate - Managed Files Source

A Backup and Migrate source for managed files.

Unlike the default "File directory" source from Backup and Migrate, this source only backup files known to Drupal (ie. with an entry in the file_managed table) and supports remote file wrapper (such as Amazon S3).

The source does not navigate the file system recursively to find files to backup, but simply query the file_managed database table. In addition to avoiding reading the file-system to collect the list of files to backup, it also avoid creating backups of non-essential files. The gain on a local file-system is likely minimal, but this can greatly helps creating backup of files from a remote file storage.