fieldMapping

<pre><code class="language-php">

<php
$this->addFieldMapping('field_blob_attachment', 'attachmentid')
    ->sourceMigration('FileBlob');
$this->addFieldMapping('field_blob_attachment:file_class')
     ->defaultValue('MigrateFileFid');
?>

</code></pre>

FileBlobMigration

<pre><code class="language-php">

<?php
class FileBlobMigration extends Migration {
public function __construct() {
   $query = db_select('legacy_file_data', 'f')
            ->fields('f', array('attachmentid', 'attachmentblob', 'filename', 'file_ownerid'));
   $this->source = new MigrateSourceSQL($query);
   $this->destination = new MigrateDestinationFile('file', 'MigrateFileBlob');
   $this->addFieldMapping('value', 'attachmentblob');
   $this->addFieldMapping('destination_file', 'filename');
   $this->addFieldMapping('uid', 'file_ownerid')
        ->sourceMigration('User');
?>

</code></pre>

gistfile1.txt

<pre><code class="language-php">

<?php
$this-addFieldMapping('field_image', 'image_filename');
$this->addFieldMapping('field_image:source_dir')
    ->defaultValue('/mnt/files');
$this->addFieldMapping('field_image:alt', 'image_alt');
$this->addFieldMapping('field_image:title', 'image_title');
?>

</code></pre>