Yes, you can change it using your method, but my .mta allows you to freely type the extension that you desire into the required field "NEW_FILE_EXTENSION", and once you run the action, it will change the extension. I believed that I explained it thoroughly in this post: Not possible to change filename w/ extension via file view - #5 by ptrk.mj , and the only difference is that I changed the string, from the excerpt, [#3], on the December.