Why...? Even if you've turned off the "Show all file extentions" (which in it self is a bad idea), then Finder is designed to show the .app suffix anyway, if you have an application with a period in it's name.
Try it. Turn off file extentions, copy an application to your desktop (or wherever), i.e. Preview.app. That will then show up as Preview. But if you rename it to Preview.jpg it will suddenly show as Preview.jpg.app, even showing file extentions are off...
This is not a new trick, people... 🙄