diff --git a/.DS_Store b/.DS_Store
index 918b049..cec08e0 100644
Binary files a/.DS_Store and b/.DS_Store differ
diff --git a/README.md b/README.md
index 300b79e..72b190c 100644
--- a/README.md
+++ b/README.md
@@ -116,7 +116,7 @@ module.exports = createConfig({
    * define list of file extensions to be considered as asset files.
    * Asset files are copied over to the build directories.
    *
-   * example, .json, .png, .jpg, .jpeg, etc
+   * default values are .json, .png, .jpg, .jpeg, .gif, .svg, etc
    */
   assetExtensions: []
 
diff --git a/bin/index.js b/bin/index.js
index c32b308..0de12c0 100755
--- a/bin/index.js
+++ b/bin/index.js
@@ -7,10 +7,36 @@ const parseBoolean = (value) => {
   return value === '' || value === true || value === 'true' || value === '1';
 };
 
-const parseList = (value) =>
-  value && typeof value === 'string'
-    ? value.split(/\s*,\s*/gim).filter(Boolean)
-    : [];
+/**
+ *
+ * @param {string} fieldName the field name
+ * @param {string[]} allowedValues - array of allowed values
+ * @returns
+ */
+const createListParser = (fieldName, allowedValues = []) => {
+  return (value) => {
+    const values =
+      value && typeof value === 'string'
+        ? value.split(/\s*,\s*/gim).filter(Boolean)
+        : [];
+
+    if (!allowedValues.length) {
+      return values;
+    }
+
+    const allowed = new Set(allowedValues);
+
+    values.forEach((value) => {
+      if (!allowed.has(value)) {
+        throw new Error(
+          `Value for ${fieldName} must be one of ${allowedValues}. ${value} is not an acceptable value`
+        );
+      }
+    });
+
+    return values;
+  };
+};
 
 const assignDefaultBooleanValue = (flags, property, defaultValue) => {
   if (typeof flags[property] === 'undefined') {
@@ -28,13 +54,13 @@ args.options([
   {
     name: 'formats',
     description: 'defines desired build formats',
-    init: parseList,
+    init: createListParser('formats', ['cjs', 'es', 'iife', 'umd']),
   },
 
   {
     name: 'envs',
     description: 'build envs',
-    init: parseList,
+    init: createListParser('envs'),
   },
 
   {
diff --git a/src/config.ts b/src/config.ts
index b7b0c0a..ff8f76e 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -18,7 +18,7 @@ export const config: Config = {
    */
   extensions: ['.js', '.ts', '.jsx', '.tsx'],
 
-  assetExtensions: ['.stories.ts'],
+  assetExtensions: [],
 
   /**
    * defines string of file patterns to process
diff --git a/src/constants.ts b/src/constants.ts
index 3e1fc0c..d696655 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -1,3 +1,8 @@
-export const REGEX_FIELDS = ['include', 'exclude'];
-
-export const PACKAGE_NAME = '@teclone/rollup-all';
+export const BASE_ASSET_EXTENSIONS = [
+  '.json',
+  '.png',
+  '.jpeg',
+  '.jpg',
+  '.gif',
+  '.svg',
+];
diff --git a/src/modules/Bundler.ts b/src/modules/Bundler.ts
index 50eedb3..5de5861 100644
--- a/src/modules/Bundler.ts
+++ b/src/modules/Bundler.ts
@@ -18,6 +18,7 @@ import { copy } from '../utils/copy';
 import { camelCase } from '../utils/camelCase';
 import { rimrafSync } from 'rimraf';
 import { forEach } from '../utils/forEach';
+import { BASE_ASSET_EXTENSIONS } from '../constants';
 
 const allExternal = () => true;
 
@@ -37,6 +38,9 @@ class Bundler {
     this.entryPath = process.cwd();
 
     const config = copy({}, defaultConfig, givenConfig);
+    config.assetExtensions = BASE_ASSET_EXTENSIONS.concat(
+      config.assetExtensions || []
+    ).map((ext) => ext.toLowerCase());
 
     // resolve entry file
     config.src = path.resolve(this.entryPath, config.src);
@@ -112,7 +116,7 @@ class Bundler {
             baseName = fileNameSegments[0];
 
             if (fileNameSegments.length > 1) {
-              extName = '.' + fileNameSegments.slice(1).join('.');
+              extName = '.' + fileNameSegments.slice(1).join('.').toLowerCase();
             }
 
             const dirName = resolvedPath;
diff --git a/src/modules/utils.ts b/src/modules/utils.ts
index eab2cab..3ccb2eb 100644
--- a/src/modules/utils.ts
+++ b/src/modules/utils.ts
@@ -46,7 +46,6 @@ export const getRollupPlugins = (opts: {
     babelPresets,
   } = opts;
   const internalNodeModulesDir = getClosestPackageDir(__dirname);
-
   const isDistBuild = format === 'umd' || format === 'iife';
 
   return [
@@ -57,7 +56,6 @@ export const getRollupPlugins = (opts: {
     commonjs({
       include: 'node_modules/**',
     }),
-
     babel({
       // we do not want to use any local babelrc
       babelrc: false,
@@ -127,7 +125,7 @@ export const getRollupPlugins = (opts: {
           : null,
       ].filter(Boolean),
 
-      // runtime for library builds and bundled for bundled builds
+      // runtime for library builds are bundled for bundled builds
       babelHelpers: isDistBuild ? 'bundled' : 'runtime',
 
       // do not process node module files, it is believed that all files in the node modules directory has been properly worked on