Source

File: wp-admin/includes/media.php

function wp_caption_input_textarea( $edit_post ) {
	// Post data is already escaped.
	$name = "attachments[{$edit_post->ID}][post_excerpt]";

	return '<textarea name="' . $name . '" id="' . $name . '">' . $edit_post->post_excerpt . '</textarea>';
}