diff --git a/lib/rails_admin/config/fields/types/dragonfly.rb b/lib/rails_admin/config/fields/types/dragonfly.rb
index <HASH>..<HASH> 100644
--- a/lib/rails_admin/config/fields/types/dragonfly.rb
+++ b/lib/rails_admin/config/fields/types/dragonfly.rb
@@ -13,7 +13,7 @@ module RailsAdmin
             bindings[:object].errors["#{name}_uid"] + bindings[:object].errors["#{name}_name"]
           end
           
-          def image?
+          register_instance_option(:image?) do
             false unless value
             if respond_to?("#{name}_name")
               super(field.send("#{name}_name"))
diff --git a/lib/rails_admin/config/fields/types/file_upload.rb b/lib/rails_admin/config/fields/types/file_upload.rb
index <HASH>..<HASH> 100644
--- a/lib/rails_admin/config/fields/types/file_upload.rb
+++ b/lib/rails_admin/config/fields/types/file_upload.rb
@@ -40,8 +40,8 @@ module RailsAdmin
             end
           end
           
-          def image?
-            (url = resource_url) && url.split('.').last =~ /jpg|jpeg|png|gif/i
+          register_instance_option :image? do
+            (url = resource_url.to_s) && url.split('.').last =~ /jpg|jpeg|png|gif/i
           end
 
           def resource_url
set image? as an instance option for easy configuration