backupformat: getFileExtension

src/backupformat.js

@@ -1,12 +1,15 @@
 'use strict';
 
 exports = module.exports = {
-    api
+    api,
 };
 
-const BoxError = require('./boxerror.js');
+const assert = require('assert'),
+    BoxError = require('./boxerror.js');
 
 function api(format) {
+    assert.strictEqual(typeof format, 'string');
+
     switch (format) {
     case 'tgz': return require('./backupformat/tgz.js');
     case 'rsync': return require('./backupformat/rsync.js');