Fix error in send_file helper (#2003)

* Fix error in send_file (mimetype_filename is not defined)

* fix formatting error message in send_file

flask/helpers.py

@@ -518,8 +518,8 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
         if mimetype is None:
             if attachment_filename is not None:
                 raise ValueError(
-                    'Unable to infer MIME-type from filename {!r}, please '
-                    'pass one explicitly.'.format(mimetype_filename)
+                    'Unable to infer MIME-type from filename {0!r}, please '
+                    'pass one explicitly.'.format(attachment_filename)
                 )
             raise ValueError(
                 'Unable to infer MIME-type because no filename is available. '

tests/test_helpers.py

@@ -398,10 +398,14 @@ class TestSendfile(object):
         with app.test_request_context():
             with pytest.raises(ValueError) as excinfo:
                 flask.send_file(StringIO("LOL"))
-
             assert 'Unable to infer MIME-type' in str(excinfo)
             assert 'no filename is available' in str(excinfo)
 
+        with app.test_request_context():
+            with pytest.raises(ValueError) as excinfo:
+                flask.send_file(StringIO("LOL"), attachment_filename='filename')
+            assert "Unable to infer MIME-type from filename 'filename'" in str(excinfo)
+
     def test_send_file_object(self):
         app = flask.Flask(__name__)