Programmatically change media author on upload

wp_change_author.php

<?php

/**
 * Add this chunk of code to your functions.php (your theme)
 * This hook will called when an attachment created.
 * http://codex.wordpress.org/Plugin_API/Action_Reference/add_attachment
 **/

add_filter( 'add_attachment', 'f77_change_author_attachment_author' );

function f77_change_author_attachment_author( $attachment_ID ) {
    // get post (attachment)
    $attachment = get_post( $attachment_ID );
    $parent = get_post( $attachment->post_parent );

    $toUpdate = array();
    $toUpdate['ID'] = $attachment_ID;
    $toUpdate['post_author'] = 38; // 38 is the user id

    // actualy changes the post_author
    wp_update_post( $toUpdate );
}

?>