Get attachment IDs for each post being exported

Currently, attachmets are only exported if you choose to export
with the 'all' option. This change will get the attachment IDs
of all of the posts being exported when the 'all' option isn't
used.

src/wp-admin/includes/export.php

@@ -144,6 +144,40 @@ function export_wp( $args = array() ) {
 	// Grab a snapshot of post IDs, just in case it changes during the export.
 	$post_ids = $wpdb->get_col( "SELECT ID FROM {$wpdb->posts} $join WHERE $where" );
 
+	// Get IDs for the attachments of each post, unless all content is already being exported.
+	if ( 'all' !== $args['content'] ) {
+		foreach ( array_chunk( $post_ids, 20 ) as $chunk ) {
+			$posts_in     = array_map( 'absint', $chunk );
+			$placeholders = array_fill( 0, count( $posts_in ), '%d' );
+
+			// Prepare the SQL statement for attachment ids
+			$attachment_ids = $wpdb->get_col(
+				$wpdb->prepare(
+					"
+				SELECT ID
+				FROM $wpdb->posts
+				WHERE post_parent IN (" . implode( ',', $placeholders ) . ") AND post_type = 'attachment'
+					",
+					$posts_in
+				)
+			);
+
+			$thumbnails_ids = $wpdb->get_col(
+				$wpdb->prepare(
+					"
+				SELECT meta_value
+				FROM {$wpdb->postmeta}
+				WHERE {$wpdb->postmeta}.post_id IN (" . implode( ',', $placeholders ) . ")
+				AND {$wpdb->postmeta}.meta_key = '_thumbnail_id'
+					",
+					$posts_in
+				)
+			);
+
+			$post_ids = array_unique( array_merge( $post_ids, $attachment_ids, $thumbnails_ids ) );
+		}
+	}
+
 	/*
 	 * Get the requested terms ready, empty unless posts filtered by category
 	 * or all content.