diff --git a/lib/cli/options.js b/lib/cli/options.js
index 8ca16d88c2..340fb01e86 100644
--- a/lib/cli/options.js
+++ b/lib/cli/options.js
@@ -10,8 +10,7 @@ const fs = require('fs');
 const yargsParser = require('yargs-parser');
 const {types, aliases} = require('./run-option-metadata');
 const {ONE_AND_DONE_ARGS} = require('./one-and-dones');
-// paranoia
-const mocharc = Object.freeze(require('../mocharc.json'));
+const mocharc = require('../mocharc.json');
 const {list} = require('./run-helpers');
 const {loadConfig, findConfig} = require('./config');
 const findUp = require('find-up');
@@ -81,11 +80,12 @@ const nargOpts = types.array
 /**
  * Wrapper around `yargs-parser` which applies our settings
  * @param {string|string[]} args - Arguments to parse
+ * @param {Object} defaultValues - Default values of mocharc.json
  * @param  {...Object} configObjects - `configObjects` for yargs-parser
  * @private
  * @ignore
  */
-const parse = (args = [], ...configObjects) => {
+const parse = (args = [], defaultValues = {}, ...configObjects) => {
   // save node-specific args for special handling.
   // 1. when these args have a "=" they should be considered to have values
   // 2. if they don't, they just boolean flags
@@ -110,6 +110,7 @@ const parse = (args = [], ...configObjects) => {
   const result = yargsParser.detailed(args, {
     configuration,
     configObjects,
+    default: defaultValues,
     coerce: coerceOpts,
     narg: nargOpts,
     alias: aliases,
@@ -322,38 +323,14 @@ const loadOptions = (argv = []) => {
     args.opts = false;
     args._ = args._.concat(optsConfig._ || []);
   }
-  // special case: "extension" option should not combine with default value.
-  // normally we want to combine "array"-type options, and we _do_ with "extension", but only
-  // within user-defined configuration (args or anything else).
-  // we must also search through any aliases of "extension" because while the arguments are ///
-  // normalized by this point, the config file values are not.
-  // only the "canonical" option name is used in `mocharc`, so we needn't worry about clearing
-  // multiple options.
-  // NOTE: as of this writing, "extension" is the only default value which is of an "array" type;
-  // it's unknown whether the the below strategy should be generalized to any other future
-  // "array"-type default option.
-  const processedMocharc = Object.assign({}, mocharc);
-  if (
-    args.extension ||
-    ['extension']
-      .concat(aliases.extension)
-      .some(
-        opt =>
-          Object.hasOwnProperty(rcConfig, opt) ||
-          Object.hasOwnProperty(pkgConfig, opt) ||
-          Object.hasOwnProperty(optsConfig, opt)
-      )
-  ) {
-    delete processedMocharc.extension;
-  }
 
   args = parse(
     args._,
+    mocharc,
     args,
     rcConfig || {},
     pkgConfig || {},
-    optsConfig || {},
-    processedMocharc
+    optsConfig || {}
   );
 
   // recombine positional arguments and "spec"
diff --git a/lib/cli/run-helpers.js b/lib/cli/run-helpers.js
index 732161b146..ed4a96da40 100644
--- a/lib/cli/run-helpers.js
+++ b/lib/cli/run-helpers.js
@@ -219,7 +219,7 @@ exports.singleRun = (mocha, {files = [], exit = false} = {}) => {
  */
 exports.watchRun = (
   mocha,
-  {extension = ['js'], grep = '', ui = 'bdd', files = []} = {}
+  {extension = [], grep = '', ui = 'bdd', files = []} = {}
 ) => {
   let runner;
 
@@ -291,7 +291,7 @@ exports.watchRun = (
  */
 exports.runMocha = (
   mocha,
-  {watch = false, extension = ['js'], grep = '', ui = 'bdd', exit = false} = {},
+  {watch = false, extension = [], grep = '', ui = 'bdd', exit = false} = {},
   files = []
 ) => {
   if (watch) {
diff --git a/lib/utils.js b/lib/utils.js
index 14c5ce126d..987b6c5adc 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -562,7 +562,6 @@ function isHiddenOnUnix(pathname) {
  *
  * @public
  * @memberof Mocha.utils
- * @todo Fix extension handling
  * @param {string} filepath - Base path to start searching from.
  * @param {string[]} extensions - File extensions to look for.
  * @param {boolean} recursive - Whether to recurse into subdirectories.
@@ -571,6 +570,7 @@ function isHiddenOnUnix(pathname) {
  * @throws {TypeError} if `filepath` is directory and `extensions` not provided.
  */
 exports.lookupFiles = function lookupFiles(filepath, extensions, recursive) {
+  extensions = extensions || [];
   var files = [];
   var stat;
 
@@ -624,7 +624,7 @@ exports.lookupFiles = function lookupFiles(filepath, extensions, recursive) {
       // ignore error
       return;
     }
-    if (!extensions) {
+    if (!extensions.length) {
       throw createMissingArgumentError(
         util.format(
           'Argument %s required when argument %s is a directory',
diff --git a/test/integration/file-utils.spec.js b/test/integration/file-utils.spec.js
index 3fe030ee46..f09018871b 100644
--- a/test/integration/file-utils.spec.js
+++ b/test/integration/file-utils.spec.js
@@ -58,7 +58,7 @@ describe('file utils', function() {
       ex.and('to have length', expectedLength);
     });
 
-    it('should parse extensions from extnsions parameter', function() {
+    it('should parse extensions from extensions parameter', function() {
       var nonJsFile = tmpFile('mocha-utils-text.txt');
       fs.writeFileSync(nonJsFile, 'yippy skippy ying yang yow');
 
@@ -66,9 +66,14 @@ describe('file utils', function() {
       expect(res, 'to contain', nonJsFile).and('to have length', 1);
     });
 
-    it('should not require the extensions parameter when looking up a file', function() {
-      var res = utils.lookupFiles(tmpFile('mocha-utils'), undefined, false);
-      expect(res, 'to be', tmpFile('mocha-utils.js'));
+    it('should require the extensions parameter when looking up a file', function() {
+      var dirLookup = function() {
+        return utils.lookupFiles(tmpFile('mocha-utils'), undefined, false);
+      };
+      expect(dirLookup, 'to throw', {
+        name: 'Error',
+        code: 'ERR_MOCHA_NO_FILES_MATCH_PATTERN'
+      });
     });
 
     it('should require the extensions parameter when looking up a directory', function() {