By default, Mail will keep file attachments that you've opened or viewed in your ~/Library/Mail Downloads folder, until you delete the associated email. You can save disk space by making the following switch:
In Mail's preferences, go to General, then set Remove unedited downloads to When Mail Quits. This does not remove the attachment from your email, just from your local cache of Mail Downloads.
On one machine I've got, I reclaimed nearly half a gigabyte after using the system for only a few months! I can't wait to see how much I get back on a system I've been using for 5 years!
[kirkmc adds: I find it odd that Mail keeps these attachments after you view them, since they're still in the emails. This is only an issue, however, if you save emails with attachments; if you delete them, then the default setting deletes the cached files as well.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20130515071439989