How to Map files if I have fid in Drupal 6 table and field in Drupal 7 content type?
I think you also need to specify the file_class:

$this-&gt;addFieldMapping('field_image', 'fid')
  -&gt;sourceMigration('ExFiles');

$this-&gt;addFieldMapping('field_image:file_class')
  -&gt;defaultValue('MigrateFileFid');


This is explained in the file migration docs; without the class, the source is assumed to be a URI and not a fid.