WP_Custom_Media_Control | Give ID to media file

The setting associated with the media control will have the ID for the attachment stored as its value. Assuming you have registered a setting for your control like this:

$wp_customize->add_setting( 'resume', array(
    'type' => 'option',
    // ...
) );

Then you would obtain the attachment ID just by reading from the option, like: get_option( 'resume' ).