diff --git a/wagtail/wagtailimages/tests/tests.py b/wagtail/wagtailimages/tests/tests.py
index 3520d01d2..cc52e6758 100644
--- a/wagtail/wagtailimages/tests/tests.py
+++ b/wagtail/wagtailimages/tests/tests.py
@@ -171,7 +171,7 @@ class TestFrontendServeView(TestCase):
 
         # Check response
         self.assertEqual(response.status_code, 200)
-        self.assertEqual(response['Content-Type'], 'image/jpeg')
+        self.assertEqual(response['Content-Type'], 'image/png')
 
     def test_get_invalid_signature(self):
         """
diff --git a/wagtail/wagtailimages/views/frontend.py b/wagtail/wagtailimages/views/frontend.py
index 6dcdbe048..a68803094 100644
--- a/wagtail/wagtailimages/views/frontend.py
+++ b/wagtail/wagtailimages/views/frontend.py
@@ -1,4 +1,5 @@
 from wsgiref.util import FileWrapper
+import imghdr
 
 from django.shortcuts import get_object_or_404
 from django.http import HttpResponse
@@ -18,6 +19,7 @@ def serve(request, signature, image_id, filter_spec):
     try:
         rendition = image.get_rendition(filter_spec)
         rendition.file.open('rb')
-        return HttpResponse(FileWrapper(rendition.file), content_type='image/jpeg')
+        image_format = imghdr.what(rendition.file)
+        return HttpResponse(FileWrapper(rendition.file), content_type='image/' + image_format)
     except InvalidFilterSpecError:
         return HttpResponse("Invalid filter spec: " + filter_spec, content_type='text/plain', status=400)