Rewrite url seo

I want to improve my urls for seo

I’m with rails attachment_fu

I want change this url

/attachments/000/234/my_great_image.jpg

like that :

/my_keywords_seo/images/my_great_image.jpg

I tried it

location ~* ^/attachments { rewrite
^/attachments/([0-9]+)/([0-9]+)/(.*)$ /my_keywords_seo/images/$3 last; }

the url is rewritten, however, the resource is not found !

However i read in Module ngx_http_rewrite_module

Flags can be any of the following:
last - completes processing of rewrite directives, after which searches
for corresponding URI and location

I don’t understand …

Somebody can help me ?

or maybe I did not understand the documentation for “last” my goal is
the url change, but once the resource has been provided … just a url
rewrite not redirecting … someone has the answer ?

nobody ?