validate the backup format

src/backups.js

@@ -26,6 +26,7 @@ exports = module.exports = {
     validatePolicy,
     validateEncryptionPassword,
     testConfig,
+    validateBackupFormat,
 
     getPolicy,
     setPolicy,
@@ -348,8 +349,6 @@ async function testConfig(backupConfig) {
     const func = storage.api(backupConfig.provider);
     if (!func) return new BoxError(BoxError.BAD_FIELD, 'unknown storage provider');
 
-    if (backupConfig.format !== 'tgz' && backupConfig.format !== 'rsync') return new BoxError(BoxError.BAD_FIELD, 'unknown format');
-
     await storage.api(backupConfig.provider).testConfig(backupConfig);
 }
 
@@ -435,6 +434,14 @@ async function setLimits(limits) {
     await settings.setJson(settings.BACKUP_LIMITS_KEY, limits);
 }
 
+function validateBackupFormat(format) {
+    assert.strictEqual(typeof format, 'string');
+
+    if (format === 'tgz' || format == 'rsync') return null;
+
+    return new BoxError(BoxError.BAD_FIELD, 'Invalid backup format');
+}
+
 async function setStorage(storageConfig) {
     assert.strictEqual(typeof storageConfig, 'object');
 
@@ -459,7 +466,10 @@ async function setStorage(storageConfig) {
         }
     }
 
-    const error = await testConfig(storageConfig);
+    let error = validateBackupFormat(storageConfig.format);
+    if (error) throw error;
+
+    error = await testConfig(storageConfig);
     if (error) throw error;
 
     if ('password' in storageConfig) { // user set password