diff --git a/django/forms/fields.py b/django/forms/fields.py
index ebc25610ee..53520530b6 100644
--- a/django/forms/fields.py
+++ b/django/forms/fields.py
@@ -692,7 +692,9 @@ class ImageField(FileField):
 
             # Annotating so subclasses can reuse it for their own validation
             f.image = image
-            f.content_type = Image.MIME[image.format]
+            # Pillow doesn't detect the MIME type of all formats. In those
+            # cases, content_type will be None.
+            f.content_type = Image.MIME.get(image.format)
         except Exception:
             # Pillow doesn't recognize it as an image.
             six.reraise(ValidationError, ValidationError(
diff --git a/docs/ref/forms/fields.txt b/docs/ref/forms/fields.txt
index c8317924d9..55a8bc049e 100644
--- a/docs/ref/forms/fields.txt
+++ b/docs/ref/forms/fields.txt
@@ -690,13 +690,16 @@ For each field, we describe the default widget used if you don't specify
     When you use an ``ImageField`` on a form, you must also remember to
     :ref:`bind the file data to the form <binding-uploaded-files>`.
 
+    After the field has been cleaned and validated, the ``UploadedFile``
+    object will have an additional ``image`` attribute containing the Pillow
+    `Image`_ instance used to check if the file was a valid image. Also,
+    ``UploadedFile.content_type`` will be updated with the image's content type
+    if Pillow can determine it, otherwise it will be set to ``None``.
+
     .. versionchanged:: 1.8
 
-        After the field has been cleaned and validated, the ``UploadedFile``
-        object will have an additional ``image`` attribute containing the Pillow
-        `Image`_ instance used to check if the file was a valid image.
-        ``UploadedFile.content_type`` is also updated with the image's content
-        type as determined by Pillow.
+        The ``image`` and ``content_type`` attributes described in the last
+        paragraph were added.
 
 .. _Pillow: http://pillow.readthedocs.org/en/latest/
 .. _Image: https://pillow.readthedocs.org/en/latest/reference/Image.html
diff --git a/docs/releases/1.8.3.txt b/docs/releases/1.8.3.txt
index c6745f92c3..ee525c9275 100644
--- a/docs/releases/1.8.3.txt
+++ b/docs/releases/1.8.3.txt
@@ -71,3 +71,6 @@ Bugfixes
   using UUID primary keys (:ticket:`24912`).
 
 * Fixed removing ``unique_together`` constraints on MySQL (:ticket:`24972`).
+
+* Fixed crash when uploading images with MIME types that Pillow doesn't detect,
+  such as bitmap, in ``forms.ImageField`` (:ticket:`24948`).
diff --git a/tests/forms_tests/tests/filepath_test_files/1x1.bmp b/tests/forms_tests/tests/filepath_test_files/1x1.bmp
new file mode 100644
index 0000000000..021d5fa928
Binary files /dev/null and b/tests/forms_tests/tests/filepath_test_files/1x1.bmp differ
diff --git a/tests/forms_tests/tests/test_fields.py b/tests/forms_tests/tests/test_fields.py
index 1d09b1a8b3..b082701c11 100644
--- a/tests/forms_tests/tests/test_fields.py
+++ b/tests/forms_tests/tests/test_fields.py
@@ -806,6 +806,26 @@ class FieldsTests(SimpleTestCase):
         self.assertEqual('PNG', uploaded_file.image.format)
         self.assertEqual('image/png', uploaded_file.content_type)
 
+    @skipIf(Image is None, "Pillow is required to test ImageField")
+    def test_imagefield_annotate_with_bitmap_image_after_clean(self):
+        """
+        This also tests the situation when Pillow doesn't detect the MIME type
+        of the image (#24948).
+        """
+        f = ImageField()
+
+        img_path = os.path.dirname(upath(__file__)) + '/filepath_test_files/1x1.bmp'
+        with open(img_path, 'rb') as img_file:
+            img_data = img_file.read()
+
+        img_file = SimpleUploadedFile('1x1.bmp', img_data)
+        img_file.content_type = 'text/plain'
+
+        uploaded_file = f.clean(img_file)
+
+        self.assertEqual('BMP', uploaded_file.image.format)
+        self.assertIsNone(uploaded_file.content_type)
+
     # URLField ##################################################################
 
     def test_urlfield_1(self):
@@ -1347,6 +1367,7 @@ class FieldsTests(SimpleTestCase):
         f.choices.sort()
         expected = [
             ('/tests/forms_tests/tests/filepath_test_files/.dot-file', '.dot-file'),
+            ('/tests/forms_tests/tests/filepath_test_files/1x1.bmp', '1x1.bmp'),
             ('/tests/forms_tests/tests/filepath_test_files/1x1.png', '1x1.png'),
             ('/tests/forms_tests/tests/filepath_test_files/directory', 'directory'),
             ('/tests/forms_tests/tests/filepath_test_files/fake-image.jpg', 'fake-image.jpg'),