Using delete_attachment to prevent an attachment not working

Updated answer for WordPress 5.5.0+

As of 5.5.0, a new filter has been added to wp_delete_attachment:

/**
 * Filters whether an attachment deletion should take place.
 *
 * @since 5.5.0
 *
 * @param bool|null $delete       Whether to go forward with deletion.
 * @param WP_Post   $post         Post object.
 * @param bool      $force_delete Whether to bypass the Trash.
 */
$check = apply_filters( 'pre_delete_attachment', null, $post, $force_delete );
if ( null !== $check ) {
    return $check;
}

This "short-circuit" filter is before almost any meaningful action is made, save for trashing the attachment if certain conditions are met. A cleaner approach would now be:

function action_maybe_delete( $id ) { 
  echo "Let's not delete Attachment id: " . $id ;
  // prevent the attachment from actually being deleted
  if( 1 == 1 ) { // made up condition 
    return false; 
  }
};
// add the action 
add_action( 'pre_delete_attachment', 'action_maybe_delete', 10, 1 ); 

• Returning anything other than null from this will prevent wp_delete_attachment from going any further, and instead the result of the filter will be returned.

Old Answer from pre 5.5.0

First, let's take a look at where delete_attachment is fired in core. This happens on line 5062 of wp-includes/post.php in the wp_delete_attachment function. Here's the relevant snippet:

/**
 * Fires before an attachment is deleted, at the start of wp_delete_attachment().
 *
 * @since 2.0.0
 *
 * @param int $post_id Attachment ID.
 */
do_action( 'delete_attachment', $post_id );

wp_delete_object_term_relationships($post_id, array('category', 'post_tag'));
wp_delete_object_term_relationships($post_id, get_object_taxonomies($post->post_type));

We can see that WordPress core is calling do_action - this method does not care what's returned; unlike apply_filters, do_action simply fires a hook and then processing continue despite any return values. Looking further at the source of wp_delete_attachment, there doesn't appear to be a way to "short-circuit" this process and prevent deleting the attachment.

Except, technically, you could use PHP's die or exit statements to end processing of the script, which should effectively prevent the file from being deleted.

function action_maybe_delete( $id ) {
    echo "Let's not delete Attachment id: " . $id ;
    // prevent the attachment from actually being deleted
    if( 1 == 1 ) { // made up condition
        die; // Prevent the script from continuing.
    }
};
// add the action
add_action( 'delete_attachment', 'action_maybe_delete', 10, 1 ); ?php