如何在.htaccess文件中使用RewriteEngine

My current .htaccess file contains this code

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} .*jpg$|.*gif$|.*png$|.*mp3$ [NC]
RewriteRule (.*) /viewed.php?file=$1

I want to change it so it rewrites the URL to /viewed.php?file=$1 ($1 being file path) if the extension is not .php, but I don't know how to do that. (I found that code on another post and added my own extensions to the list)

If someone could link me to an explanation of how Rewrite Conditions work that would be great because I don't understand how to make them.

Rewrite all, if not .php

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !^.*\.php$ [NC]
RewriteRule (.*) /viewed.php?file=$1 [L]

Documentation: http://httpd.apache.org/docs/current/mod/mod_rewrite.html

Learn RegEx: https://regexone.com/