diff --git src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
index 813de1c..7ee3f9c 100644
--- src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
@@ -122,6 +122,9 @@ class WP_REST_Attachments_Controller extends WP_REST_Posts_Controller {
 		$type = $file['type'];
 		$file = $file['file'];
 
+		/** Require the image processing functions. */
+		require_once ABSPATH . 'wp-admin/includes/image.php';
+
 		// use image exif/iptc data for title and caption defaults if possible
 		$image_meta = wp_read_image_metadata( $file );