Fixed #21548 -- Added FileExtensionValidator and validate_image_file_extension.

docs/ref/validators.txt

@@ -279,3 +279,30 @@ to, or in lieu of custom ``field.clean()`` methods.
       ``decimal_places``.
     - ``'max_whole_digits'`` if the number of whole digits is larger than
       the difference between ``max_digits`` and ``decimal_places``.
+
+``FileExtensionValidator``
+--------------------------
+
+.. class:: FileExtensionValidator(allowed_extensions, message, code)
+
+    .. versionadded:: 1.11
+
+    Raises a :exc:`~django.core.exceptions.ValidationError` with a code of
+    ``'invalid_extension'`` if the ``value`` cannot be found in
+    ``allowed_extensions``.
+
+    .. warning::
+
+        Don't rely on validation of the file extension to determine a file's
+        type. Files can be renamed to have any extension no matter what data
+        they contain.
+
+``validate_image_file_extension``
+---------------------------------
+
+.. data:: validate_image_file_extension
+
+    .. versionadded:: 1.11
+
+    Uses Pillow to ensure that the ``value`` is `a valid image extension
+    <https://pillow.readthedocs.org/en/latest/handbook/image-file-formats.html>`_.

docs/releases/1.11.txt

@@ -192,6 +192,9 @@ Models
   <django.db.models.query.QuerySet.update_or_create>` and
   :meth:`~django.db.models.query.QuerySet.get_or_create`.
 
+* :class:`~django.db.models.ImageField` now has a default
+  :data:`~django.core.validators.validate_image_file_extension` validator.
+
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~
 
@@ -237,7 +240,10 @@ URLs
 Validators
 ~~~~~~~~~~
 
-* ...
+* Added :class:`~django.core.validators.FileExtensionValidator` to validate
+  file extensions and
+  :data:`~django.core.validators.validate_image_file_extension` to validate
+  image files.
 
 .. _backwards-incompatible-1.11: