change the method for assuming s3

services/filestore/app/coffee/PersistorManager.coffee

@@ -2,9 +2,13 @@ settings = require("settings-sharelatex")
 logger = require("logger-sharelatex")
 S3PersistorManager = require("./S3PersistorManager")
 
+# assume s3 if none specified
+settings.filestoreBackend ||= "s3"
+
+
 logger.log backend:settings.filestoreBackend, "Loading backend"
 module.exports = switch settings.filestoreBackend
-	when "s3",null
+	when "s3"
 		S3PersistorManager
 	else
 		throw new Error( "Unknown filestore backend: #{settings.filestoreBackend}" )