diff --git a/README.md b/README.md
index f7f8d92..16406f4 100644
--- a/README.md
+++ b/README.md
@@ -97,10 +97,10 @@
 - `requirements-txt-fixer` - Sorts entries in requirements.txt and removes incorrect entry for `pkg-resources==0.0.0`
 - `sort-simple-yaml` - Sorts simple YAML files which consist only of top-level keys, preserving comments and blocks.
 - `trailing-whitespace` - Trims trailing whitespace.
-    - Markdown linebreak trailing spaces preserved for `.md` and`.markdown`;
-      use `args: ['--markdown-linebreak-ext=txt,text']` to add other extensions,
-      `args: ['--markdown-linebreak-ext=*']` to preserve them for all files,
-      or `args: ['--no-markdown-linebreak-ext']` to disable and always trim.
+    - To preserve Markdown [hard linebreaks](https://github.github.com/gfm/#hard-line-break)
+      use `args: [--markdown-linebreak-ext=md]` (or other extensions used
+      by your markdownfiles).  If for some reason you want to treat all files
+      as markdown, use `--markdown-linebreak-ext=*`.
 
 ### As a standalone package
 
diff --git a/pre_commit_hooks/trailing_whitespace_fixer.py b/pre_commit_hooks/trailing_whitespace_fixer.py
index 062f6e3..1b54fbd 100644
--- a/pre_commit_hooks/trailing_whitespace_fixer.py
+++ b/pre_commit_hooks/trailing_whitespace_fixer.py
@@ -35,24 +35,25 @@
     parser = argparse.ArgumentParser()
     parser.add_argument(
         '--no-markdown-linebreak-ext',
-        action='store_const',
-        const=[],
-        default=argparse.SUPPRESS,
-        dest='markdown_linebreak_ext',
-        help='Do not preserve linebreak spaces in Markdown',
+        action='store_true',
+        help=argparse.SUPPRESS,
     )
     parser.add_argument(
         '--markdown-linebreak-ext',
         action='append',
-        const='',
-        default=['md,markdown'],
+        default=[],
         metavar='*|EXT[,EXT,...]',
-        nargs='?',
-        help='Markdown extensions (or *) for linebreak spaces',
+        help=(
+            'Markdown extensions (or *) to not strip linebreak spaces.  '
+            'default: %(default)s'
+        ),
     )
     parser.add_argument('filenames', nargs='*', help='Filenames to fix')
     args = parser.parse_args(argv)
 
+    if args.no_markdown_linebreak_ext:
+        print('--no-markdown-linebreak-ext now does nothing!')
+
     md_args = args.markdown_linebreak_ext
     if '' in md_args:
         parser.error('--markdown-linebreak-ext requires a non-empty argument')
@@ -66,7 +67,7 @@
     for ext in md_exts:
         if any(c in ext[1:] for c in r'./\:'):
             parser.error(
-                "bad --markdown-linebreak-ext extension '{}' (has . / \\ :)\n"
+                "bad --markdown-linebreak-ext extension {!r} (has . / \\ :)\n"
                 "  (probably filename; use '--markdown-linebreak-ext=EXT')"
                 .format(ext),
             )
diff --git a/tests/trailing_whitespace_fixer_test.py b/tests/trailing_whitespace_fixer_test.py
index 7ee9e63..82c9b6d 100644
--- a/tests/trailing_whitespace_fixer_test.py
+++ b/tests/trailing_whitespace_fixer_test.py
@@ -14,7 +14,7 @@
     ),
 )
 def test_fixes_trailing_whitespace(input_s, expected, tmpdir):
-    path = tmpdir.join('file.txt')
+    path = tmpdir.join('file.md')
     path.write(input_s)
     assert main((path.strpath,)) == 1
     assert path.read() == expected
@@ -36,89 +36,39 @@
     assert ret == 0
 
 
-def test_markdown_ok(tmpdir):
-    filename = tmpdir.join('foo.md')
-    filename.write_binary(b'foo  \n')
-    ret = main((filename.strpath,))
-    assert filename.read_binary() == b'foo  \n'
-    assert ret == 0
-
-
-# filename, expected input, expected output
-MD_TESTS_1 = (
-    ('foo.md', 'foo  \nbar \n  ', 'foo  \nbar\n'),
-    ('bar.Markdown', 'bar   \nbaz\t\n\t\n', 'bar  \nbaz\n\n'),
-    ('.md', 'baz   \nquux  \t\n\t\n', 'baz\nquux\n\n'),
-    ('txt', 'foo   \nbaz \n\t\n', 'foo\nbaz\n\n'),
-)
-
-
-@pytest.mark.parametrize(('filename', 'input_s', 'output'), MD_TESTS_1)
-def test_fixes_trailing_markdown_whitespace(filename, input_s, output, tmpdir):
-    path = tmpdir.join(filename)
-    path.write(input_s)
-    ret = main([path.strpath])
+@pytest.mark.parametrize('ext', ('md', 'Md', '.md', '*'))
+def test_fixes_markdown_files(tmpdir, ext):
+    path = tmpdir.join('test.md')
+    path.write(
+        'foo  \n'  # leaves alone
+        'bar \n'  # less than two so it is removed
+        'baz    \n'  # more than two so it becomes two spaces
+        '\t\n'  # trailing tabs are stripped anyway
+        '\n  ',  # whitespace at the end of the file is removed
+    )
+    ret = main((path.strpath, '--markdown-linebreak-ext={}'.format(ext)))
     assert ret == 1
-    assert path.read() == output
+    assert path.read() == (
+        'foo  \n'
+        'bar\n'
+        'baz  \n'
+        '\n'
+        '\n'
+    )
 
 
-# filename, expected input, expected output
-MD_TESTS_2 = (
-    ('foo.txt', 'foo  \nbar \n  \n', 'foo  \nbar\n\n'),
-    ('bar.Markdown', 'bar   \nbaz\t\n\t\n', 'bar  \nbaz\n\n'),
-    ('bar.MD', 'bar   \nbaz\t   \n\t\n', 'bar  \nbaz  \n\n'),
-    ('.txt', 'baz   \nquux  \t\n\t\n', 'baz\nquux\n\n'),
-    ('txt', 'foo   \nbaz \n\t\n', 'foo\nbaz\n\n'),
-)
-
-
-@pytest.mark.parametrize(('filename', 'input_s', 'output'), MD_TESTS_2)
-def test_markdown_linebreak_ext_opt(filename, input_s, output, tmpdir):
-    path = tmpdir.join(filename)
-    path.write(input_s)
-    ret = main(('--markdown-linebreak-ext=TxT', path.strpath))
-    assert ret == 1
-    assert path.read() == output
-
-
-# filename, expected input, expected output
-MD_TESTS_3 = (
-    ('foo.baz', 'foo  \nbar \n  ', 'foo  \nbar\n'),
-    ('bar', 'bar   \nbaz\t\n\t\n', 'bar  \nbaz\n\n'),
-)
-
-
-@pytest.mark.parametrize(('filename', 'input_s', 'output'), MD_TESTS_3)
-def test_markdown_linebreak_ext_opt_all(filename, input_s, output, tmpdir):
-    path = tmpdir.join(filename)
-    path.write(input_s)
-    # need to make sure filename is not treated as argument to option
-    ret = main(('--markdown-linebreak-ext=*', path.strpath))
-    assert ret == 1
-    assert path.read() == output
-
-
-@pytest.mark.parametrize(('arg'), ('--', 'a.b', 'a/b'))
+@pytest.mark.parametrize('arg', ('--', 'a.b', 'a/b', ''))
 def test_markdown_linebreak_ext_badopt(arg):
     with pytest.raises(SystemExit) as excinfo:
         main(['--markdown-linebreak-ext', arg])
     assert excinfo.value.code == 2
 
 
-# filename, expected input, expected output
-MD_TESTS_4 = (
-    ('bar.md', 'bar   \nbaz\t   \n\t\n', 'bar\nbaz\n\n'),
-    ('bar.markdown', 'baz   \nquux  \n', 'baz\nquux\n'),
-)
-
-
-@pytest.mark.parametrize(('filename', 'input_s', 'output'), MD_TESTS_4)
-def test_no_markdown_linebreak_ext_opt(filename, input_s, output, tmpdir):
-    path = tmpdir.join(filename)
-    path.write(input_s)
-    ret = main(['--no-markdown-linebreak-ext', path.strpath])
-    assert ret == 1
-    assert path.read() == output
+def test_prints_warning_with_no_markdown_ext(capsys, tmpdir):
+    f = tmpdir.join('f').ensure()
+    assert main((f.strpath, '--no-markdown-linebreak-ext')) == 0
+    out, _ = capsys.readouterr()
+    assert out == '--no-markdown-linebreak-ext now does nothing!\n'
 
 
 def test_preserve_non_utf8_file(tmpdir):