diff --git a/django/db/models/fields/files.py b/django/db/models/fields/files.py
index e0c095974dc..b0dce381a61 100644
--- a/django/db/models/fields/files.py
+++ b/django/db/models/fields/files.py
@@ -242,7 +242,11 @@ class FileField(Field):
         # (ie. upload_to='path/to/upload/dir'), the length of the generated
         # name equals the length of the uploaded name plus a constant. Thus
         # we can tell the user how much shorter the name should be (roughly).
-        length = len(self.generate_filename(model_instance, value.name))
+        if value and value._committed:
+            filename = value.name
+        else:
+            filename = self.generate_filename(model_instance, value.name)
+        length = len(filename)
         if self.max_length and length > self.max_length:
             error_values = {'extra': length - self.max_length}
             raise ValidationError(self.error_messages['max_length'] % error_values)
diff --git a/tests/modeltests/files/models.py b/tests/modeltests/files/models.py
index 41344727485..cefc7c7244a 100644
--- a/tests/modeltests/files/models.py
+++ b/tests/modeltests/files/models.py
@@ -26,5 +26,5 @@ class Storage(models.Model):
 
     normal = models.FileField(storage=temp_storage, upload_to='tests')
     custom = models.FileField(storage=temp_storage, upload_to=custom_upload_to)
-    random = models.FileField(storage=temp_storage, upload_to=random_upload_to)
+    random = models.FileField(storage=temp_storage, upload_to=random_upload_to, max_length=16)
     default = models.FileField(storage=temp_storage, upload_to='tests', default='tests/default.txt')
diff --git a/tests/modeltests/files/tests.py b/tests/modeltests/files/tests.py
index 3e256f787f4..565d4c89423 100644
--- a/tests/modeltests/files/tests.py
+++ b/tests/modeltests/files/tests.py
@@ -5,6 +5,7 @@ import shutil
 import tempfile
 
 from django.core.cache import cache
+from django.core.exceptions import ValidationError
 from django.core.files import File
 from django.core.files.base import ContentFile
 from django.core.files.uploadedfile import SimpleUploadedFile
@@ -102,11 +103,23 @@ class FileStorageTests(TestCase):
         obj4.random.save("random_file", ContentFile(b"random content"))
         self.assertTrue(obj4.random.name.endswith("/random_file"))
 
-        # Clean up the temporary files and dir.
-        obj1.normal.delete()
-        obj2.normal.delete()
-        obj3.default.delete()
-        obj4.random.delete()
+    def test_max_length(self):
+        """
+        Test that FileField validates the length of the generated file name
+        that will be stored in the database. Regression for #9893.
+        """
+        # upload_to = 'unused', so file names are saved as '456/xxxxx'.
+        # max_length = 16, so names longer than 12 characters are rejected.
+        s1 = Storage(random=SimpleUploadedFile(12 * 'x', b"content"))
+        s1.full_clean()
+        with self.assertRaises(ValidationError):
+            Storage(random=SimpleUploadedFile(13 * 'x', b"content")).full_clean()
+
+        # Ticket #18515: validation for an already saved file should not check
+        # against a regenerated file name (and potentially raise a ValidationError
+        # if max_length is exceeded
+        s1.save()
+        s1.full_clean()
 
 
 class FileTests(unittest.TestCase):
diff --git a/tests/regressiontests/model_fields/tests.py b/tests/regressiontests/model_fields/tests.py
index 5d3d42ef2a5..a89ffcae320 100644
--- a/tests/regressiontests/model_fields/tests.py
+++ b/tests/regressiontests/model_fields/tests.py
@@ -365,15 +365,3 @@ class FileFieldTests(unittest.TestCase):
         field = d._meta.get_field('myfile')
         field.save_form_data(d, 'else.txt')
         self.assertEqual(d.myfile, 'else.txt')
-
-    def test_max_length(self):
-        """
-        Test that FileField validates the length of the generated file name
-        that will be stored in the database. Regression for #9893.
-
-        """
-        # upload_to = 'unused', so file names are saved as 'unused/xxxxx'.
-        # max_length = 100, so names longer than 93 characters are rejected.
-        Document(myfile=93 * 'x').full_clean()
-        with self.assertRaises(ValidationError):
-            Document(myfile=94 * 'x').full_clean()