Yes, you should make an attachment.php template and put your adsense code there.
When you upload a file, you’re actually making a sort of pretend post. This attachment post lives in the posts table, just like real posts, drafts, private posts, and Pages. It just has a status of “attachment”.
The content of this attachment post is actually information about the file. Specifically, the description you gave to the file in the upload form is there, along with an IMG tag for the image. There’s also a bunch of stuff in the post’s custom fields, such as the size of the post, its location, the thumbnail’s location, etc…
So when you click through to an attachment on a page, you’re actually seeing that attachment post.
If all you want to do is to change the description displayed under the image on its page, just hit Browse All, click the image, click the Edit link, and change the description there. Then Save.
If you want to add anything more complex and want to do it universally across all attachment pages, copy the theme’s index.php or single.php file into attachment.php, then edit that file to add your Adsense code where you want it.
