Migrating files with descriptions
Here is a link that shows all the arguments you can pass to MigrateFileFieldHandler. As you can see description is the 7th argument.

If you don't want to specify any of the arguments before description, you still have to pass NULL to them e.g.:

$arguments = MigrateFileFieldHandler::arguments(
  drupal_get_path('module', 'my_migrate').'/docs', 
  'file_copy', 
  FILE_EXISTS_RENAME, 
  NULL, 
  NULL, 
  NULL, 
  array('source_field' =&gt; 'migrate_source_description_field'), //Can also pass a string here
);