Limit ACF field to edit only

block-file.php

<?php
if (isset($block['data']['preview_image_help'])):
    $image = get_template_directory_uri() . '/blocks/previews/' . basename(__FILE__, '.php') . '.png';
    echo '<img src="' . $image . '" style="width:100%; height:auto;">';
else:
?>
  Front End Content
<?php
endif; // preview_image_help 
?>

functions.php

   add_action('acf/init', 'block_blockname');
    function block_blockname() {

        // check function exists.
        if( function_exists('acf_register_block_type') ) {

            acf_register_block_type(array(
                'name'              => 'blockname',
                'title'             => __('Block Name'),
                'icon' => '<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24"><path d="M22 0h-20v6h1.999c0-1.174.397-3 2.001-3h4v16.874c0 1.174-.825 2.126-2 2.126h-1v2h9.999v-2h-.999c-1.174 0-2-.952-2-2.126v-16.874h4c1.649 0 2.02 1.826 2.02 3h1.98v-6z"/></svg>',
                'render_template'   => 'blocks/block-file.php',
                'category'          => 'XYZ',
                'mode'                => 'edit',
                'supports' => array(
                    'align' => false,
                    'multiple' => true,
                ),
                'example'  => array(
                    'attributes' => array(
                        'mode' => 'preview',
                        'data' => array(
                            'preview_image_help' => true,
                        )
                    )
                )
            ));
        }
    }