Attachment custom fields

index.php

<?php 

namespace Utkarsh\Attachment\Meta;

const META_NAME = 'media_credit';

/**
 * Bootstrap.
 */
function bootstrap() {
	add_filter( 'attachment_fields_to_edit', __NAMESPACE__ . '\\attachment_custom_fields', 10, 2 );
	add_filter( 'attachment_fields_to_save', __NAMESPACE__ . '\\attachment_fields_save', 10, 2 );
}

/**
 * Add attachment additional fields.
 *
 * @param array   $form_fields Fields.
 * @param WP_Post $post Post object.
 *
 * @return array modified form fields.
 */
function attachment_custom_fields( array $form_fields, WP_Post $post ): array {
	$form_fields[ META_NAME ] = [
		'label' => esc_html__( 'Credit', 'domain' ),
		'input' => 'text',
		'value' => get_post_meta( $post->ID, META_NAME, true ),
	];
	return $form_fields;
}

/**
 * Attachment save custom fields.
 *
 * @param array $post Post array.
 * @param array $attachment Attachment data.
 *
 * @return array Post array.
 */
function attachment_fields_save( array $post, array $attachment ): array {
	if ( isset( $attachment[ META_NAME ] ) ) {
		update_post_meta( $post['ID'], META_NAME, sanitize_text_field( $attachment[ META_NAME ] ) );
	}
	return $post;
}