WP - register custom field for media

register_media_field.php

<?php 
function be_attachment_field_credit( $form_fields, $post ) {
	// 
	//  ---- CS ---
	//  
       $form_fields['media_date'] = array(
		'label' => 'Datum',
		'input' => 'text',
		'value' => get_post_meta( $post->ID, 'media_date', true ),
		// 'helps' => 'datace',
	);

	return $form_fields;
}

add_filter( 'attachment_fields_to_edit', 'be_attachment_field_credit', 10, 2 );


/**
 * Save media values
 *
 * @param $post array, the post data for database
 * @param $attachment array, attachment fields from $_POST form
 * @return $post array, modified post data
 */

function be_attachment_field_credit_save( $post, $attachment ) {

	if( isset( $attachment['media_date'] ) )
		update_post_meta( $post['ID'], 'media_date', esc_attr( $attachment['media_date'] ) );

	return $post;
}

add_filter( 'attachment_fields_to_save', 'be_attachment_field_credit_save', 10, 2 );