--- a/python/mozlint/mozlint/pathutils.py
+++ b/python/mozlint/mozlint/pathutils.py
@@ -21,16 +21,20 @@ class FilterPath(object):
     def finder(self):
         if self._finder:
             return self._finder
         self._finder = FileFinder(
             self.path, find_executables=False, ignore=self.exclude)
         return self._finder
 
     @property
+    def ext(self):
+        return os.path.splitext(self.path)[1]
+
+    @property
     def exists(self):
         return os.path.exists(self.path)
 
     @property
     def isfile(self):
         return os.path.isfile(self.path)
 
     @property
@@ -88,19 +92,27 @@ def filterpaths(paths, linter, **lintarg
     # Paths with and without globs will be handled separately,
     # pull them apart now.
     includepaths = [p for p in include if p.exists]
     excludepaths = [p for p in exclude if p.exists]
 
     includeglobs = [p for p in include if not p.exists]
     excludeglobs = [p for p in exclude if not p.exists]
 
+    extensions = linter.get('extensions')
     keep = set()
     discard = set()
     for path in map(FilterPath, paths):
+        # Exclude bad file extensions
+        if extensions and path.isfile and path.ext not in extensions:
+            continue
+
+        if path.match(excludeglobs):
+            continue
+
         # First handle include/exclude directives
         # that exist (i.e don't have globs)
         for inc in includepaths:
             # Only excludes that are subdirectories of the include
             # path matter.
             excs = [e for e in excludepaths if inc.contains(e)]
 
             if path.contains(inc):
@@ -124,22 +136,21 @@ def filterpaths(paths, linter, **lintarg
         # Next handle include/exclude directives that
         # contain globs.
         if path.isfile:
             # If the specified path is a file it must be both
             # matched by an include directive and not matched
             # by an exclude directive.
             if not path.match(includeglobs):
                 continue
-            elif path.match(excludeglobs):
-                continue
+
             keep.add(path)
         elif path.isdir:
             # If the specified path is a directory, use a
             # FileFinder to resolve all relevant globs.
-            path.exclude = excludeglobs
+            path.exclude = [e.path for e in excludeglobs]
             for pattern in includeglobs:
                 for p, f in path.finder.find(pattern.path):
                     keep.add(path.join(p))
 
     # Only pass paths we couldn't exclude here to the underlying linter
     lintargs['exclude'] = [f.path for f in discard]
     return [f.path for f in keep]

new file mode 100644
--- /dev/null
+++ b/python/mozlint/test/files/foobar.py
@@ -0,0 +1,2 @@
+# Oh no.. we called this variable foobar, bad!
+foobar = "a string"

--- a/python/mozlint/test/linters/external.lint
+++ b/python/mozlint/test/linters/external.lint
@@ -17,14 +17,14 @@ def lint(files, **lintargs):
                         LINTER, path=path, lineno=i+1, column=1, rule="no-foobar"))
     return results
 
 
 LINTER = {
     'name': "ExternalLinter",
     'description': "It's bad to have the string foobar in js files.",
     'include': [
-        '**/*.js',
-        '**/*.jsm',
+        'python/mozlint/test/files',
     ],
     'type': 'external',
+    'extensions': ['.js', '.jsm'],
     'payload': lint,
 }

--- a/python/mozlint/test/test_roller.py
+++ b/python/mozlint/test/test_roller.py
@@ -15,18 +15,18 @@ from mozlint.errors import LintersNotCon
 here = os.path.abspath(os.path.dirname(__file__))
 
 
 class TestLintRoller(TestCase):
 
     def __init__(self, *args, **kwargs):
         TestCase.__init__(self, *args, **kwargs)
 
-        filedir = os.path.join(here, 'files')
-        self.files = [os.path.join(filedir, f) for f in os.listdir(filedir)]
+        self.filedir = os.path.join(here, 'files')
+        self.files = [os.path.join(self.filedir, f) for f in os.listdir(self.filedir)]
         self.lintdir = os.path.join(here, 'linters')
 
         names = ('string.lint', 'regex.lint', 'external.lint')
         self.linters = [os.path.join(self.lintdir, n) for n in names]
 
     def setUp(self):
         TestCase.setUp(self)
         self.lint = LintRoller()
@@ -65,11 +65,16 @@ class TestLintRoller(TestCase):
     def test_roll_with_excluded_path(self):
         self.lint.lintargs.update({'exclude': ['**/foobar.js']})
 
         self.lint.read(self.linters)
         result = self.lint.roll(self.files)
 
         self.assertEqual(len(result), 0)
 
+    def test_roll_with_invalid_extension(self):
+        self.lint.read(os.path.join(self.lintdir, 'external.lint'))
+        result = self.lint.roll(os.path.join(self.filedir, 'foobar.py'))
+        self.assertEqual(len(result), 0)
+
 
 if __name__ == '__main__':
     main()

--- a/python/mozlint/test/test_types.py
+++ b/python/mozlint/test/test_types.py
@@ -66,13 +66,13 @@ class TestLinterTypes(TestCase):
 
     def test_no_filter(self):
         self.lint.read(os.path.join(self.lintdir, 'explicit_path.lint'))
         result = self.lint.roll(self.files)
         self.assertEqual(len(result), 0)
 
         self.lint.lintargs['use_filters'] = False
         result = self.lint.roll(self.files)
-        self.assertEqual(len(result), 1)
+        self.assertEqual(len(result), 2)
 
 
 if __name__ == '__main__':
     main()

--- a/tools/lint/docs/create.rst
+++ b/tools/lint/docs/create.rst
@@ -48,16 +48,17 @@ Each ``.lint`` file must have a variable
 linter. Here are the supported keys:
 
 * name - The name of the linter (required)
 * description - A brief description of the linter's purpose (required)
 * type - One of 'string', 'regex' or 'external' (required)
 * payload - The actual linting logic, depends on the type (required)
 * include - A list of glob patterns that must be matched (optional)
 * exclude - A list of glob patterns that must not be matched (optional)
+* extensions - A list of file extensions to be considered (optional)
 * setup - A function that sets up external dependencies (optional)
 
 In addition to the above, some ``.lint`` files correspond to a single lint rule. For these, the
 following additional keys may be specified:
 
 * message - A string to print on infraction (optional)
 * hint - A string with a clue on how to fix the infraction (optional)
 * rule - An id string for the lint rule (optional)

--- a/tools/lint/flake8.lint
+++ b/tools/lint/flake8.lint
@@ -40,16 +40,17 @@ LINE_OFFSETS = {
     'E302': (-2, 3),
 }
 """Maps a flake8 error to a lineoffset tuple.
 
 The offset is of the form (lineno_offset, num_lines) and is passed
 to the lineoffset property of `ResultContainer`.
 """
 
+EXTENSIONS = ['.py', '.lint']
 results = []
 
 
 def process_line(line):
     try:
         res = json.loads(line)
     except ValueError:
         return
@@ -130,11 +131,12 @@ LINTER = {
         'taskcluster',
         'testing/firefox-ui',
         'testing/marionette/client',
         'testing/puppeteer',
         'testing/talos/',
         'tools/lint',
     ],
     'exclude': [],
+    'extensions': EXTENSIONS,
     'type': 'external',
     'payload': lint,
 }