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 43602)
+++ src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php	(working copy)
@@ -805,6 +805,9 @@
 			return new WP_Error( 'rest_upload_file_too_big', sprintf( __( 'This file is too big. Files must be less than %s KB in size.' ), get_site_option( 'fileupload_maxk', 1500 ) ), array( 'status' => 400 ) );
 		}
 
+		// Include admin function to get access to upload_is_user_over_quota().
+		require_once ABSPATH . 'wp-admin/includes/ms.php';
+
 		if ( upload_is_user_over_quota( false ) ) {
 			return new WP_Error( 'rest_upload_user_quota_exceeded', __( 'You have used your space quota. Please delete files before uploading.' ), array( 'status' => 400 ) );
 		}