diff --git a/httpie/sessions.py b/httpie/sessions.py
index 5a4acef8..cc54def6 100644
--- a/httpie/sessions.py
+++ b/httpie/sessions.py
@@ -77,7 +77,9 @@ class Session(BaseConfigDict):
             if value is None:
                 continue  # Ignore explicitly unset headers
 
-            value = value.decode('utf8')
+            if type(value) is not str:
+                value = value.decode('utf8')
+
             if name.lower() == 'user-agent' and value.startswith('HTTPie/'):
                 continue
 
diff --git a/tests/test_sessions.py b/tests/test_sessions.py
index 4fbc0031..ea22a1ad 100644
--- a/tests/test_sessions.py
+++ b/tests/test_sessions.py
@@ -16,6 +16,7 @@ from httpie.sessions import Session
 from httpie.utils import get_expired_cookies
 from tests.test_auth_plugins import basic_auth
 from utils import HTTP_OK, MockEnvironment, http, mk_config_dir
+from fixtures import FILE_PATH_ARG
 
 
 class SessionTestBase:
@@ -161,6 +162,12 @@ class TestSession(SessionTestBase):
         assert 'Content-Type' not in r2.json['headers']
         assert 'If-Unmodified-Since' not in r2.json['headers']
 
+    def test_session_with_upload(self, httpbin):
+        self.start_session(httpbin)
+        r = http('--session=test', '--form', '--verbose', 'POST', httpbin.url + '/post',
+                 f'test-file@{FILE_PATH_ARG}', 'foo=bar', env=self.env())
+        assert HTTP_OK in r
+
     def test_session_by_path(self, httpbin):
         self.start_session(httpbin)
         session_path = self.config_dir / 'session-by-path.json'