Index: src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
===================================================================
--- src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php	(revision 43848)
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php	(working copy)
@@ -173,6 +173,7 @@
 		do_action( 'rest_insert_attachment', $attachment, $request, true );
 
 		// Include admin function to get access to wp_generate_attachment_metadata().
+		require_once ABSPATH . 'wp-admin/includes/media.php';
 		require_once ABSPATH . 'wp-admin/includes/image.php';
 
 		wp_update_attachment_metadata( $id, wp_generate_attachment_metadata( $id, $file ) );