RewriteEngine on
RewriteBase /
RewriteRule ^/?f/([a-zA-Z0-9_]+)$ /file/?i=$1 [L]
RewriteRule ^/?d/([a-zA-Z0-9_]+)$ /download/?i=$1 [L]
RewriteRule ^/?p/([a-zA-Z0-9_]+)$ /preview/?i=$1 [L]