Source

File: wp-includes/link-template.php

function get_attachment_link( $post = null, $leavename = false ) {
	global $wp_rewrite;

	$link = false;

	$post   = get_post( $post );
	$parent = ( $post->post_parent > 0 && $post->post_parent != $post->ID ) ? get_post( $post->post_parent ) : false;
	if ( $parent && ! in_array( $parent->post_type, get_post_types(), true ) ) {
		$parent = false;
	}

	if ( $wp_rewrite->using_permalinks() && $parent ) {
		if ( 'page' === $parent->post_type ) {
			$parentlink = _get_page_link( $post->post_parent ); // Ignores page_on_front.
		} else {
			$parentlink = get_permalink( $post->post_parent );
		}

		if ( is_numeric( $post->post_name ) || false !== strpos( get_option( 'permalink_structure' ), '%category%' ) ) {
			$name = 'attachment/' . $post->post_name; // <permalink>/<int>/ is paged so we use the explicit attachment marker.
		} else {
			$name = $post->post_name;
		}

		if ( strpos( $parentlink, '?' ) === false ) {
			$link = user_trailingslashit( trailingslashit( $parentlink ) . '%postname%' );
		}

		if ( ! $leavename ) {
			$link = str_replace( '%postname%', $name, $link );
		}
	} elseif ( $wp_rewrite->using_permalinks() && ! $leavename ) {
		$link = home_url( user_trailingslashit( $post->post_name ) );
	}

	if ( ! $link ) {
		$link = home_url( '/?attachment_id=' . $post->ID );
	}

	/**
	 * Filters the permalink for an attachment.
	 *
	 * @since 2.0.0
	 *
	 * @param string $link    The attachment's permalink.
	 * @param int    $post_id Attachment ID.
	 */
	return apply_filters( 'attachment_link', $link, $post->ID );
}