Use .gz extension for attachment filename if it is supplied

app/models/dunlop/target_file_model.rb

@@ -7,7 +7,7 @@ module Dunlop::TargetFileModel
     if attachment.persisted?
       {}
     else
-      {'Content-Disposition' => %|attachment; filename="#{attachment.target_basename}"|} # can be resource based and only available on initialization. Therefore persisted?
+      {'Content-Disposition' => %|attachment; filename="#{File.basename(attachment.local_target_filename)}"|} # can be resource based and only available on initialization. Therefore persisted?
     end
   end
 
@@ -64,8 +64,10 @@ module Dunlop::TargetFileModel
   end
 
   def local_target_filename
-    result = File.join(Rails.application.config.working_path, target_basename)
-    gzip_file? ? result + ".gz" : result
+    @local_target_filename ||= begin
+      result = File.join(Rails.application.config.working_path, target_basename)
+      gzip_file? ? result + ".gz" : result
+    end
   end
 
   # Target file basename used on creation to deliver the desired file_name.
@@ -85,7 +87,7 @@ module Dunlop::TargetFileModel
     # hook to implement post generation actions like sending by email and/or to other systems by means of web call and/or to create a transfer instance
   end
 
-  # Implement in Subclass. Example:
+  # Implement #data or this method in Subclass. Example:
   #   CsvBuilder::BladiBla.new(resource).data
   def generate_file
     working_file.puts builder.data