Fix fallback to custom s3 buckets

services/filestore/app/js/MigrationPersistor.js

@@ -115,7 +115,7 @@ module.exports = function(primary, fallback) {
   }
 
   function _getFallbackBucket(bucket) {
-    return Settings.filestore.fallback.buckets[bucket]
+    return Settings.filestore.fallback.buckets[bucket] || bucket
   }
 
   function _wrapFallbackMethod(method) {

services/filestore/test/acceptance/js/FilestoreTests.js

@@ -374,7 +374,7 @@ describe('Filestore', function() {
         })
       })
 
-      if (backend === 'S3Persistor') {
+      if (backend === 'S3Persistor' || backend === 'FallbackGcsToS3Persistor') {
         describe('with a file in a specific bucket', function() {
           let constantFileContent, fileId, fileUrl, bucketName