The php page of the website can be opened, but what is the reason why html, txt, etc. cannot be opened? css and js open normally

The php page of the website can be opened, but what is the reason why html, txt, etc. cannot be opened? css and js open normally
lnmpa environment

Firefox prompts like this:

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The php page of the website can be opened, but what is the reason why html, txt, etc. cannot be opened? css and js open normally
lnmpa environment

Firefox prompts like this:

First check whether the URL is correct, then check whether the file exists, and then check the permissions. (In fact, the easiest way is to directly check the server log)

It may be redirected, because css and js are static resources, so they are not redirected.
If the server used is apache, then check .htaccess in the root directory. Add html, txt Go in