htaccess to display markdown only if file exists

.htaccess

#Turn on Rewrite
RewriteEngine On

#Check if requested file ends in .md or .markdown
RewriteCond $1 \.(md|markdown)$ [NC]

#Check if file exists
RewriteCond %{REQUEST_FILENAME} -f

#Rewrite so $_GET['file'] is available to PHP code
RewriteRule ^(.*)$ /index.php?file=$1 [L]

index.php

<?php
/*
For files in web root:
index.php
file.md
file2.md
**********************************/

/*
Allows users such as: http://mysite.com/file.md
Which would effectively become: http://mysite.com/index.php?file=file.md
**********************************/

/*
Files which do not exist will give a 404 error
http://mysite.com/some_random_file.md #Yields 404 error
http://mysite.com/index.php?file.md #Works - Still need to check in code if file_exists();
**********************************/