There’s a couple things you could do. If we follow the Template Hierarchy you could rename your content-attachment.php file to attachment.php and remove image.php. This will ensure that the image view will fallback to attachment.php.
If you want to use content-attachment.php then you could use the template_include hook to tell WordPress to load in which situation.
/**
* Tell WordPress to load a specific template under specific circumstances.
*
* @param String $template_path
*
* @return String $template_path
*/
add_filter( 'template_include', function( $template_path ) {
if( false !== strpos( $template_path, 'image.php' ) ) {
// Path to your PHP file, relative to the twentysixteen theme (or child theme)
// This would mean `content-attachmetn.php` is in the twentysixteen root, with page.php and singe.php
$template_path = get_template_part( 'content-attachment' );
}
return $template_path;
} );