diff --git a/wagtail/wagtailimages/models.py b/wagtail/wagtailimages/models.py
index cffc27377a..10b0727dd6 100644
--- a/wagtail/wagtailimages/models.py
+++ b/wagtail/wagtailimages/models.py
@@ -249,7 +249,7 @@ class AbstractImage(CollectionMember, TagSearchable):
             )
         except Rendition.DoesNotExist:
             # Generate the rendition image
-            generated_image, output_format = filter.run(self, BytesIO())
+            generated_image = filter.run(self, BytesIO())
 
             # Generate filename
             input_filename = os.path.basename(self.file.name)
@@ -262,7 +262,7 @@ class AbstractImage(CollectionMember, TagSearchable):
                 'gif': '.gif',
             }
 
-            output_extension = filter.spec.replace('|', '.') + FORMAT_EXTENSIONS[output_format]
+            output_extension = filter.spec.replace('|', '.') + FORMAT_EXTENSIONS[generated_image.format_name]
             if cache_key:
                 output_extension = cache_key + '.' + output_extension
 
@@ -273,7 +273,7 @@ class AbstractImage(CollectionMember, TagSearchable):
             rendition, created = self.renditions.get_or_create(
                 filter=filter,
                 focal_point_key=cache_key,
-                defaults={'file': File(generated_image, name=output_filename)}
+                defaults={'file': File(generated_image.f, name=output_filename)}
             )
 
         return rendition
@@ -387,8 +387,6 @@ class Filter(models.Model):
             for operation in self.operations:
                 willow = operation.run(willow, image) or willow
 
-            output_format = original_format
-
             if original_format == 'jpeg':
                 # Allow changing of JPEG compression quality
                 if hasattr(settings, 'WAGTAILIMAGES_JPEG_QUALITY'):
@@ -396,22 +394,18 @@ class Filter(models.Model):
                 else:
                     quality = 85
 
-                willow.save_as_jpeg(output, quality=quality)
+                return willow.save_as_jpeg(output, quality=quality)
             elif original_format == 'gif':
                 # Convert image to PNG if it's not animated
                 if not willow.has_animation():
-                    output_format = 'png'
-                    willow.save_as_png(output)
+                    return willow.save_as_png(output)
                 else:
-                    willow.save_as_gif(output)
+                    return willow.save_as_gif(output)
             elif original_format == 'bmp':
                 # Convert to PNG
-                output_format = 'png'
-                willow.save_as_png(output)
+                return willow.save_as_png(output)
             else:
-                willow.save(original_format, output)
-
-        return output, output_format
+                return willow.save(original_format, output)
 
     def get_cache_key(self, image):
         vary_parts = []
diff --git a/wagtail/wagtailimages/views/images.py b/wagtail/wagtailimages/views/images.py
index 5f2179c44c..21012963ab 100644
--- a/wagtail/wagtailimages/views/images.py
+++ b/wagtail/wagtailimages/views/images.py
@@ -211,8 +211,9 @@ def preview(request, image_id, filter_spec):
     image = get_object_or_404(get_image_model(), id=image_id)
 
     try:
-        response, image_format = Filter(spec=filter_spec).run(image, HttpResponse())
-        response['Content-Type'] = 'image/' + image_format
+        response = HttpResponse()
+        image = Filter(spec=filter_spec).run(image, response)
+        response['Content-Type'] = 'image/' + image.format_name
         return response
     except InvalidFilterSpecError:
         return HttpResponse("Invalid filter spec: " + filter_spec, content_type='text/plain', status=400)