diff --git a/index.js b/index.js
index 803af30..c99b6ab 100644
--- a/index.js
+++ b/index.js
@@ -161,7 +161,7 @@ module.exports.lint = function lint(indexes, options, callback) {
     callback(null,
       formatLint(hierarchy(
         inputs
-          .filter(filterJS)
+          .filter(filterJS(options.extension))
           .reduce(function (memo, file) {
             return memo.concat(parseFn(file));
           }, [])
diff --git a/lib/filter_js.js b/lib/filter_js.js
index 8370983..1d98ecf 100644
--- a/lib/filter_js.js
+++ b/lib/filter_js.js
@@ -1,9 +1,11 @@
 'use strict';
 
+var path = require('path');
+
 /**
  * Node & browserify support requiring JSON files. JSON files can't be documented
  * with JSDoc or parsed with espree, so we filter them out before
- * they reach documentation's machinery. 
+ * they reach documentation's machinery.
  * This creates a filter function for use with Array.prototype.filter, which
  * expect as argument a file as an objectg with the 'file' property
  *
@@ -13,22 +15,15 @@
  * is in the extension whitelist
  */
 function filterJS(extensions) {
-  if (!(extensions instanceof Array)) {
-    extensions = typeof extensions === 'string' ? [extensions] : [];
-  }
 
-  extensions.push('js');
+  extensions = extensions || [];
+  if (typeof extensions === 'string') {
+    extensions = [extensions];
+  }
+  extensions = extensions.concat('js');
 
   return function (data) {
-    var extension;
-    for (var i = 0; i < extensions.length; i++) {
-      extension = extensions[i];
-      if (data.file.slice(-(extension.length + 1)) === '.' + extension) {
-        return true;
-      }
-    }
-
-    return false;
+    return extensions.indexOf(path.extname(data.file).substring(1)) !== -1;
   };
 }
 
diff --git a/test/bin.js b/test/bin.js
index 1d78088..f133023 100644
--- a/test/bin.js
+++ b/test/bin.js
@@ -96,6 +96,15 @@ test('external modules option', function (t) {
   });
 });
 
+test('extension option', function (t) {
+  documentation(['build fixture/extension/jsx.jsx ' +
+    '--extension=jsx'], function (err, data) {
+    t.ifError(err);
+    t.equal(data.length, 1, 'includes jsx file');
+    t.end();
+  });
+});
+
 test('invalid arguments', function (group) {
   group.test('bad -f option', function (t) {
     documentation(['build -f DOES-NOT-EXIST fixture/internal.input.js'], function (err) {
diff --git a/test/fixture/extension/jsx.jsx b/test/fixture/extension/jsx.jsx
new file mode 100644
index 0000000..0027ec3
--- /dev/null
+++ b/test/fixture/extension/jsx.jsx
@@ -0,0 +1,4 @@
+/**
+ * apples
+ */
+function apples() {}