Ignore missing files and report a 404 status

.htaccess

# add this code to your .htaccess file (Apache based servers only)
<IfModule mod_rewrite.c>
	RewriteEngine On
	RewriteBase /
	RewriteCond %{REQUEST_FILENAME} !-f
	RewriteCond %{REQUEST_URI} \.(jpg|jpeg|png|gif|bmp|ico|css|js|swf|htm|html|txt|php|asp|aspx)$ [NC]
	RewriteRule .* - [L,R=404]
</IfModule>

Use this code if your web server is based on NGINX:
location ~* \.(gif|jpg|jpeg|png|css|js|svg|txt|zip|html|json|aspx|xml)$ { error_page 404 /404.html; expires max; access_log off; }
Don't forget to create the static 404.html file!