Image resize fails when Froce flag is set

mongoengine/fields.py

@@ -1108,6 +1108,7 @@ class ImageGridFsProxy(GridFSProxy):
 
         try:
             img = Image.open(file_obj)
+            img_format = img.format
         except:
             raise ValidationError('Invalid image')
 
@@ -1142,20 +1143,20 @@ class ImageGridFsProxy(GridFSProxy):
 
         if thumbnail:
             thumb_id = self._put_thumbnail(thumbnail,
-                                          img.format)
+                                          img_format)
         else:
             thumb_id = None
 
         w, h = img.size
 
         io = StringIO()
-        img.save(io, img.format)
+        img.save(io, img_format)
         io.seek(0)
 
         return super(ImageGridFsProxy, self).put(io,
                                                  width=w,
                                                  height=h,
-                                                 format=img.format,
+                                                 format=img_format,
                                                  thumbnail_id=thumb_id,
                                                  **kwargs)
 

tests/test_fields.py

@@ -1851,6 +1851,29 @@ class FieldTest(unittest.TestCase):
 
         t.image.delete()
 
+    def test_image_field_resize_force(self):
+        if PY3:
+            raise SkipTest('PIL does not have Python 3 support')
+
+        class TestImage(Document):
+            image = ImageField(size=(185, 37, True))
+
+        TestImage.drop_collection()
+
+        t = TestImage()
+        t.image.put(open(TEST_IMAGE_PATH, 'r'))
+        t.save()
+
+        t = TestImage.objects.first()
+
+        self.assertEquals(t.image.format, 'PNG')
+        w, h = t.image.size
+
+        self.assertEquals(w, 185)
+        self.assertEquals(h, 37)
+
+        t.image.delete()
+
     def test_image_field_thumbnail(self):
         if PY3:
             raise SkipTest('PIL does not have Python 3 support')