diff --git a/src/openforms/registrations/contrib/json/plugin.py b/src/openforms/registrations/contrib/json/plugin.py
index bff7f72a62..76fd78b151 100644
--- a/src/openforms/registrations/contrib/json/plugin.py
+++ b/src/openforms/registrations/contrib/json/plugin.py
@@ -1,3 +1,5 @@
+import base64
+
 from django.utils.translation import gettext_lazy as _
 
 from openforms.submissions.models import Submission
@@ -26,7 +28,8 @@ def register_submission(self, submission: Submission, options: OptionsT) -> None
                     continue
                 options["form_variables"].remove(attachment.form_key)
                 with attachment.content.open("rb") as f:
-                    values[attachment.form_key] = f.read()
+                    f.seek(0)
+                    values[attachment.form_key] = base64.b64encode(f.read()).decode()
 
         # TODO-4908: what should the behaviour be when a form
         #  variable is not in the data or static variables?
diff --git a/src/openforms/registrations/contrib/json/tests/test_backend.py b/src/openforms/registrations/contrib/json/tests/test_backend.py
index fb4c8d0dbc..71025c4b41 100644
--- a/src/openforms/registrations/contrib/json/tests/test_backend.py
+++ b/src/openforms/registrations/contrib/json/tests/test_backend.py
@@ -61,7 +61,7 @@ def test_submission_with_json_backend(self):
             "values": {
                 "firstName": "We Are",
                 "lastName": "Checking",
-                "file": b"This is example content.",
+                "file": "VGhpcyBpcyBleGFtcGxlIGNvbnRlbnQu",
                 "auth_bsn": "123456789",
             }
         }