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Creating text file with new extension does not give supported option until the finish button is pressed. Until then the checkbox is grayed out. However, pressing the finish button, but not releasing un-grays the button and moving my mouse away from the button causes the finish action to not be triggered. This allow me to select click the 'Add supported extension' checkbox.
I think it is in text module.
Try to press tab key when you have written a new extension to move focus to another component in the panel. I think it is designed behaviour. I set it as INVALID.