test_input.php

<?php
add_filter( 'attachment_fields_to_edit', 'attachment_fields_to_edit', 10, 2 );
add_filter( 'attachment_fields_to_save', 'attachment_fields_to_save', 10, 2 );

function attachment_fields_to_edit( $form_fields, $post ) {
	$form_fields["client_test_input"] = array(
		"label" => __( 'Test Input URL' , 'client' ),
		"value" => get_post_meta( $post->ID, "_client_test_input", true ),
        "helps" => __( 'Test Input Field', 'client' )
	);

	return $form_fields;
}

function attachment_fields_to_save( $post, $attachment ) {
	if( empty( $attachment['client_test_input'] ) )
		delete_post_meta( $post['ID'], '_client_test_input' );
	else
		update_post_meta( $post['ID'], '_client_test_input', esc_url( $attachment['client_test_input'] ) );

	return $post;
}