Not to change the file type directly. Suffix (Resolved)
The reason: the suffix lead to hidden default file name suffix, that is no matter how useless, the default suffix still there, just not displayed.
Solution:
Just open a file, top left, click View => In the fourth column you can see the file extension => check either
Picture guide
The first step to open the file, look at the top left, click View
The second step check the file extension
