media_library_permissions.php

/*-----------------------------------------------------------------------------------*/
/* Media Library Permissions */
/*-----------------------------------------------------------------------------------*/


add_action('pre_get_posts','users_own_attachments');

function users_own_attachments( $wp_query_obj )
{
    global $current_user, $pagenow;

    if( !is_a( $current_user, 'WP_User') )
        return;

    if( 'upload.php' != $pagenow && 'media-upload.php' != $pagenow)
        return;

    if( !current_user_can('delete_pages') )
        $wp_query_obj->set('author', $current_user->ID );

    return;
}