In outlook 2007 , It will not download full attachment in IMAP server, by default. User may experience corrupt file when attachment file size is large. User can setup outlook 2007 to download full attachment.

1. Click on Tools -> Send/Receive -> Send / Receive Settings -> Define Send/Receive Groups 

Click on 'All Accounts' , then click on button 'Edit'

Click on option 'Receive mail items' and 'Download complete items including attachments for subscribed folders'