Currently we define the filename postfix starts from the first dot, we can change it to be from the last dot in future version.
May I ask the purpose why you want to edit the filename postfix? Since the touchscreen only recognizes Gcode and Data file.
I uploaded a firmware file to Local Storage using ideaMakers upload function. Apparently ideaMaker always adds ".gcode" to the uploaded filenames, even though the file already has a ".firm" postfix. So that was the reason I tried to rename the filename postfix using Raise Touch, but it ended up in total chaos, because I had dots in the filename, and everything after the first dot wasn't visible in the TextBox.
This is not a big deal though