diff --git a/CHANGES.rst b/CHANGES.rst
index fd4e0533..a0a9cd28 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -9,6 +9,12 @@ Version 1.0.3
 
 Unreleased
 
+-   :func:`send_file` encodes filenames as ASCII instead of Latin-1
+    (ISO-8859-1). This fixes compatibility with Gunicorn, which is
+    stricter about header encodings than PEP 3333. (`#2766`_)
+
+.. _#2766: https://github.com/pallets/flask/issues/2766
+
 
 Version 1.0.2
 -------------
diff --git a/flask/helpers.py b/flask/helpers.py
index df0b91fc..7679a496 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -506,6 +506,10 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
 
     .. _RFC 2231: https://tools.ietf.org/html/rfc2231#section-4
 
+    .. versionchanged:: 1.0.3
+        Filenames are encoded with ASCII instead of Latin-1 for broader
+        compatibility with WSGI servers.
+
     :param filename_or_fp: the filename of the file to send.
                            This is relative to the :attr:`~Flask.root_path`
                            if a relative path is specified.
@@ -564,11 +568,11 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
                             'sending as attachment')
 
         try:
-            attachment_filename = attachment_filename.encode('latin-1')
+            attachment_filename = attachment_filename.encode('ascii')
         except UnicodeEncodeError:
             filenames = {
                 'filename': unicodedata.normalize(
-                    'NFKD', attachment_filename).encode('latin-1', 'ignore'),
+                    'NFKD', attachment_filename).encode('ascii', 'ignore'),
                 'filename*': "UTF-8''%s" % url_quote(attachment_filename),
             }
         else:
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index b3535b28..ae1c0805 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -638,15 +638,22 @@ class TestSendfile(object):
         assert options['filename'] == 'index.txt'
         rv.close()
 
-    def test_attachment_with_utf8_filename(self, app, req_ctx):
-        rv = flask.send_file('static/index.html', as_attachment=True, attachment_filename=u'Ñandú／pingüino.txt')
-        content_disposition = set(rv.headers['Content-Disposition'].split('; '))
-        assert content_disposition == set((
-            'attachment',
-            'filename="Nandu/pinguino.txt"',
-            "filename*=UTF-8''%C3%91and%C3%BA%EF%BC%8Fping%C3%BCino.txt"
-        ))
+    @pytest.mark.usefixtures('req_ctx')
+    @pytest.mark.parametrize(('filename', 'ascii', 'utf8'), (
+        ('index.html', 'index.html', False),
+        (u'Ñandú／pingüino.txt', '"Nandu/pinguino.txt"',
+        '%C3%91and%C3%BA%EF%BC%8Fping%C3%BCino.txt'),
+        (u'Vögel.txt', 'Vogel.txt', 'V%C3%B6gel.txt'),
+    ))
+    def test_attachment_filename_encoding(self, filename, ascii, utf8):
+        rv = flask.send_file('static/index.html', as_attachment=True, attachment_filename=filename)
         rv.close()
+        content_disposition = rv.headers['Content-Disposition']
+        assert 'filename=%s' % ascii in content_disposition
+        if utf8:
+            assert "filename*=UTF-8''" + utf8 in content_disposition
+        else:
+            assert "filename*=UTF-8''" not in content_disposition
 
     def test_static_file(self, app, req_ctx):
         # default cache timeout is 12 hours