Delete Stored Offline Attachments in Mail.app
If your mail account is IMAP or Exchange, you can simply rebuild your mailbox(es).  


In Mail.app, ensure that the Mailbox sidebar is exposed and highlighted (click the mailbox you would like to rebuild)  
On the menu bar, select Mailbox -&gt; Rebuild to rebuild the highlighted mailbox. 


Repeat, if necessary for any other mailboxes.