RIN67630:
No, if I try to rename the tab from "a_setup" to e.g. "a_setup.ino" I get an error that "a_setup.ino" already exists.
Try this.
Create a new sketch and save it. Now look in the sketch folder. What files() are there ?
Add a new tab and name it a_setup. Do not give it a file extension.
Save the sketch. Now look in the sketch folder. What files() are there now ?
Now do you see why you cannot rename a_setup to a_setup.ino ? You could, however, rename it to a_setup.h or somethingelse.ino