From c02bda429983e5461a65a5ec4cab2b29420ace52 Mon Sep 17 00:00:00 2001
From: Gyanu <gyanum.ug20.cse@nitp.ac.in>
Date: Fri, 28 Aug 2026 09:34:59 +0530
Subject: [PATCH] Treat empty uploads as None when File(allow_none=True).

Empty strings, JSON null, and FileStorage without a filename were rejected even though allow_none was set.
---
 CHANGELOG.rst                   | 8 ++++++++
 src/flask_marshmallow/fields.py | 9 ++++++++-
 tests/test_fields.py            | 9 +++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e5c9481..2702b40 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -1,6 +1,14 @@
 Changelog
 =========
 
+Unreleased
+**********
+
+Bug fixes:
+
+* Treat empty or missing uploads as ``None`` when ``File(allow_none=True)``
+  (:issue:`319`).
+
 1.5.0 (2026-04-15)
 ******************
 
diff --git a/src/flask_marshmallow/fields.py b/src/flask_marshmallow/fields.py
index 1b6c53b..e8af15f 100755
--- a/src/flask_marshmallow/fields.py
+++ b/src/flask_marshmallow/fields.py
@@ -233,7 +233,14 @@ def deserialize(
         data: typing.Mapping[str, typing.Any] | None = None,
         **kwargs,
     ):
-        if isinstance(value, Sequence) and len(value) == 0:
+        from werkzeug.datastructures import FileStorage
+
+        if self.allow_none:
+            if isinstance(value, str) and value in ("", "null"):
+                value = None
+            elif isinstance(value, FileStorage) and not value.filename:
+                value = None
+        elif isinstance(value, Sequence) and len(value) == 0:
             value = missing
         return super().deserialize(value, attr, data, **kwargs)
 
diff --git a/tests/test_fields.py b/tests/test_fields.py
index e399a02..0acfd78 100644
--- a/tests/test_fields.py
+++ b/tests/test_fields.py
@@ -168,6 +168,15 @@ def test_file_field(ma, mockauthor):
         field.deserialize("123", mockauthor)
 
 
+def test_file_field_allow_none(ma):
+    field = ma.File(allow_none=True)
+    assert field.deserialize(None) is None
+    assert field.deserialize("") is None
+    assert field.deserialize("null") is None
+    empty = FileStorage(io.BytesIO(b""), "")
+    assert field.deserialize(empty) is None
+
+
 def test_config_field(ma, app, mockauthor):
     app.config["NAME"] = "test"
     field = ma.Config(key="NAME")