--- a/python/mozlint/mozlint/parser.py
+++ b/python/mozlint/mozlint/parser.py
@@ -36,16 +36,19 @@ class Parser(object):
             raise LinterParseError(linter['path'], "Invalid type '{}'".format(linter['type']))
 
         for attr in ('include', 'exclude'):
             if attr in linter and (not isinstance(linter[attr], list) or
                                    not all(isinstance(a, basestring) for a in linter[attr])):
                 raise LinterParseError(linter['path'], "The {} directive must be a "
                                                        "list of strings!".format(attr))
 
+        if 'extensions' in linter:
+            linter['extensions'] = [e.strip('.') for e in linter['extensions']]
+
     def parse(self, path):
         """Read a linter and return its LINTER definition.
 
         :param path: Path to the linter.
         :returns: List of linter definitions ([dict])
         :raises: LinterNotFound, LinterParseError
         """
         if not os.path.isfile(path):

--- a/python/mozlint/mozlint/pathutils.py
+++ b/python/mozlint/mozlint/pathutils.py
@@ -22,17 +22,17 @@ class FilterPath(object):
         if self._finder:
             return self._finder
         self._finder = FileFinder(
             self.path, ignore=self.exclude)
         return self._finder
 
     @property
     def ext(self):
-        return os.path.splitext(self.path)[1]
+        return os.path.splitext(self.path)[1].strip('.')
 
     @property
     def exists(self):
         return os.path.exists(self.path)
 
     @property
     def isfile(self):
         return os.path.isfile(self.path)

--- a/python/mozlint/test/linters/string.yml
+++ b/python/mozlint/test/linters/string.yml
@@ -1,8 +1,9 @@
 StringLinter:
     description: Make sure the string foobar never appears in browser js files because it is bad
     rule: no-foobar
     include:
         - '**/*.js'
         - '**/*.jsm'
     type: string
+    extensions: ['.js', 'jsm']
     payload: foobar

--- a/python/mozlint/test/test_parser.py
+++ b/python/mozlint/test/test_parser.py
@@ -30,16 +30,18 @@ def test_parse_valid_linter(parse):
     assert len(lintobj) == 1
 
     lintobj = lintobj[0]
     assert isinstance(lintobj, dict)
     assert 'name' in lintobj
     assert 'description' in lintobj
     assert 'type' in lintobj
     assert 'payload' in lintobj
+    assert 'extensions' in lintobj
+    assert set(lintobj['extensions']) == set(['js', 'jsm'])
 
 
 @pytest.mark.parametrize('linter', [
     'invalid_type.yml',
     'invalid_extension.ym',
     'invalid_include.yml',
     'invalid_exclude.yml',
     'missing_attrs.yml',

--- a/tools/lint/flake8.yml
+++ b/tools/lint/flake8.yml
@@ -13,11 +13,11 @@ flake8:
         - testing/mochitest
         - testing/talos/
         - tools/git
         - tools/lint
         - tools/mercurial
         - toolkit/components/telemetry
     exclude:
         - testing/mochitest/pywebsocket
-    extensions: ['.py']
+    extensions: ['py']
     type: external
     payload: flake8_:lint