diff --git a/wagtail/wagtailimages/fields.py b/wagtail/wagtailimages/fields.py
index ef768e837..10d076bd4 100644
--- a/wagtail/wagtailimages/fields.py
+++ b/wagtail/wagtailimages/fields.py
@@ -76,7 +76,7 @@ class WagtailImageField(ImageField):
     def to_python(self, data):
         f = super(WagtailImageField, self).to_python(data)
 
-        self.check_image_file_format(f)
         self.check_image_file_size(f)
+        self.check_image_file_format(f)
 
         return f