How to check the existence of a file using .htaccess?

Question

htaccess

The site receives a request of the form site.xxx/path/file.AAA redirect)? As I can see. If the file does not exist then RewriteRule ^(.*)\.ААА$ _engine.html?error [QSA,L] if there is then RewriteRule ^(.*)\.ААА$ _engine.html?$1 [QSA,L] (this is working rule) but can't make RewriteCond conditions. Ap1 It's easier, code: RewriteCond %{REQUEST_FILENAME} (.*)\.ААА RewriteCond %1.BBB !-f RewriteRule ^ _engine.html?404 [QSA,L] Works if there is an error in the name, but does not work if there is an error on my way