diff --git a/django/core/files/storage.py b/django/core/files/storage.py
index 7590171259..73126a3cb9 100644
--- a/django/core/files/storage.py
+++ b/django/core/files/storage.py
@@ -25,16 +25,11 @@ class Storage(object):
     # The following methods represent a public interface to private methods.
     # These shouldn't be overridden by subclasses unless absolutely necessary.
 
-    def open(self, name, mode='rb', mixin=None):
+    def open(self, name, mode='rb'):
         """
-        Retrieves the specified file from storage, using the optional mixin
-        class to customize what features are available on the File returned.
+        Retrieves the specified file from storage.
         """
-        file = self._open(name, mode)
-        if mixin:
-            # Add the mixin as a parent class of the File returned from storage.
-            file.__class__ = type(mixin.__name__, (mixin, file.__class__), {})
-        return file
+        return self._open(name, mode)
 
     def save(self, name, content):
         """
diff --git a/docs/releases/1.4.txt b/docs/releases/1.4.txt
index 94c1721992..90c5e24e9d 100644
--- a/docs/releases/1.4.txt
+++ b/docs/releases/1.4.txt
@@ -527,6 +527,30 @@ This functionality has been removed due to intractable performance and
 security issues. Any existing usage of ``verify_exists`` should be
 removed.
 
+``django.core.files.storage.Storage.open``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``open`` method of the base Storage class took an obscure parameter
+``mixin`` which allowed to dynamically change the base classes of the
+returned file object. In the rare case you relied on the `mixin` parameter,
+you can easily achieve the same by overriding the `open` method, e.g.::
+
+    from django.core.files import File
+    from django.core.files.storage import FileSystemStorage
+
+    class Spam(File):
+        """
+        Spam, spam, spam, spam and spam.
+        """
+        def ham(self):
+            return 'eggs'
+
+    class SpamStorage(FileSystemStorage):
+        """
+        A custom file storage backend.
+        """
+        def open(self, name, mode='rb'):
+            return Spam(open(self.path(name), mode))
 
 .. _deprecated-features-1.4:
 
diff --git a/tests/regressiontests/file_storage/tests.py b/tests/regressiontests/file_storage/tests.py
index c50ad91e2f..f00d502931 100644
--- a/tests/regressiontests/file_storage/tests.py
+++ b/tests/regressiontests/file_storage/tests.py
@@ -231,26 +231,6 @@ class FileStorageTests(unittest.TestCase):
         self.storage.base_url = None
         self.assertRaises(ValueError, self.storage.url, 'test.file')
 
-    def test_file_with_mixin(self):
-        """
-        File storage can get a mixin to extend the functionality of the
-        returned file.
-        """
-        self.assertFalse(self.storage.exists('test.file'))
-
-        class TestFileMixin(object):
-            mixed_in = True
-
-        f = ContentFile('custom contents')
-        f_name = self.storage.save('test.file', f)
-
-        self.assertTrue(isinstance(
-            self.storage.open('test.file', mixin=TestFileMixin),
-            TestFileMixin
-        ))
-
-        self.storage.delete('test.file')
-
     def test_listdir(self):
         """
         File storage returns a tuple containing directories and files.