When using Exchange Server with Thunderbird clients over IMAP, you might get problems with wrong attchments in Thunderbird. However you will be able to open the attchments correctly with Outlook Web Access. Unfortunately Microsoft changed the message size which is a RFC violation. Now there are two options to correct this:

1. Changing the client side:

Do the follwoing on each client:

• Go to Tools->Options or Edit->Preferences
• Go to ‘Advanced’ Tab
• Click ‘Config Editor’ button
• Enter ‘mail.server.server’ as the filter
• Enter the name mail.server.server<number>.fetch_by_chunks, and set it to
  ‘false’ (eg i have mail.server.server3.fetch_by_chunks=false).

2. Change the server side for all clients:

• Start the Exchange Administrator program.
• Select <Organization>, and then <Site>. Select Configuration, and then select the Protocols container.
• Double-click the “IMAP4 (Mail) Site Defaults” object.
• Clear the “Enable fast message retrieval” check box.

http://support.microsoft.com/kb/191504

For Exchange servers > 2003:

You can enable the exact size for everyone with:
Set-ImapSettings -EnableExactRFC822Size:$true

Or only for a specific user:
Set-CASMailbox “IMAP User” -ImapUseProtocolDefaults:$false -ImapEnableExactRFC822Size:$true