Treat empty uploads as None when File(allow_none=True)

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)
 ******************
 

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)
 

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")