What is the alternative to `` when it comes to calling Media (image) files in the 'attachment.php' file?

Here's one way to do it:

<?php if ( wp_attachment_is_image( $post->id ) ) : $att_image = wp_get_attachment_image_src( $post->id, "full"); ?>

<a href="<?php echo wp_get_attachment_url($post->id); ?>" title="<?php the_title(); ?>" rel="attachment">
<img src="<?php echo $att_image[0];?>" width="<?php echo $att_image[1];?>" height="<?php echo $att_image[2];?>" alt="" />
</a>

<?php endif; ?>

Use it inside the loop. Above example first checks if the attachment is an image.

Or you might also want to just use wp_get_attachment_image() to output the entire image tag:

https://developer.wordpress.org/reference/functions/wp_get_attachment_image/

There's also get_atached_media that you can use.

https://codex.wordpress.org/Function_Reference/get_attached_media

Calling the_content from within attachment.php will bring you the Attachment Description.

Put the code below on your page and see what you can get from within attachment.php file:

while ( have_posts() ) : the_post();

    /**
     * @link    https://codex.wordpress.org/Function_Reference/wp_get_attachment_metadata#Related
     */
    $data = array(
        get_the_ID(), // Gets att ID.
        get_permalink(), // Gets att page link.
        get_the_title(), // Gets att title.
        get_the_content(), // Gets att description.
        wp_get_attachment_metadata(), // Gets att metadata.
    );

    var_dump( $data );

endwhile;

Also, you might want to take a look at https://codex.wordpress.org/Function_Reference/wp_get_attachment_metadata#Related and see other functions that will perhaps be useful for you.