diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 2776c3f9..293a32df 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -6,6 +6,11 @@ This document records all notable changes to `HTTPie <https://httpie.org>`_.
 This project adheres to `Semantic Versioning <https://semver.org/>`_.
 
 
+`2.1.0-dev`_ (unreleased)
+-------------------------
+* Fixed ``--form`` file upload mixed with redirected ``stdin`` error handling.
+
+
 `2.0.0`_ (2020-01-12)
 -------------------------
 * Removed Python 2.7 support (`EOL Jan 2020 <https://www.python.org/doc/sunset-python-2/>`_).
diff --git a/httpie/__init__.py b/httpie/__init__.py
index c5f7ac47..eee945f8 100644
--- a/httpie/__init__.py
+++ b/httpie/__init__.py
@@ -3,6 +3,6 @@ HTTPie - a CLI, cURL-like tool for humans.
 
 """
 
-__version__ = '2.0.0'
+__version__ = '2.1.0-dev'
 __author__ = 'Jakub Roztocil'
 __licence__ = 'BSD'
diff --git a/httpie/cli/argparser.py b/httpie/cli/argparser.py
index 4924552e..cf096d8d 100644
--- a/httpie/cli/argparser.py
+++ b/httpie/cli/argparser.py
@@ -246,7 +246,8 @@ class HTTPieArgumentParser(argparse.ArgumentParser):
         if self.args.data:
             self.error('Request body (from stdin or a file) and request '
                        'data (key=value) cannot be mixed. Pass '
-                       '--ignore-stdin to let key/value take priority.')
+                       '--ignore-stdin to let key/value take priority. '
+                       'See https://httpie.org/doc#scripting for details.')
         self.args.data = getattr(fd, 'buffer', fd).read()
 
     def _guess_method(self):
diff --git a/tests/test_httpie.py b/tests/test_httpie.py
index 252b0a17..70bc2f9d 100644
--- a/tests/test_httpie.py
+++ b/tests/test_httpie.py
@@ -98,6 +98,27 @@ def test_POST_file(httpbin_both):
     assert FILE_CONTENT in r
 
 
+def test_form_POST_file_redirected_stdin(httpbin):
+    """
+    <https://github.com/jakubroztocil/httpie/issues/840>
+
+    """
+    with open(FILE_PATH) as f:
+        r = http(
+            '--form',
+            'POST',
+            httpbin + '/post',
+            'file@' + FILE_PATH,
+            tolerate_error_exit_status=True,
+            env=MockEnvironment(
+                stdin=f,
+                stdin_isatty=False,
+            ),
+        )
+    assert r.exit_status == ExitStatus.ERROR
+    assert 'cannot be mixed' in r.stderr
+
+
 def test_headers(httpbin_both):
     r = http('GET', httpbin_both + '/headers', 'Foo:bar')
     assert HTTP_OK in r
diff --git a/tests/test_uploads.py b/tests/test_uploads.py
index 81298fa5..b0c7fc5d 100644
--- a/tests/test_uploads.py
+++ b/tests/test_uploads.py
@@ -3,6 +3,7 @@ import os
 import pytest
 
 from httpie.cli.exceptions import ParseError
+from httpie.status import ExitStatus
 from utils import MockEnvironment, http, HTTP_OK
 from fixtures import FILE_PATH_ARG, FILE_PATH, FILE_CONTENT
 
@@ -77,4 +78,5 @@ class TestRequestBodyFromFilePath:
             env=env,
             tolerate_error_exit_status=True,
         )
+        assert r.exit_status == ExitStatus.ERROR
         assert 'cannot be mixed' in r.stderr