Comment 45 for bug 220991

This problem also occurs in an update/upgrade process from Thunderbird 3.x to higher versions (as far as up from 3.0.x to 3.1)
The root cause is that the variable
mail.imap.fetch_by_chunks no longer exists in 3.1 (and later versions 6.0 -10.0) and the new variable
mail.server.default.fetch_by_chunks is set by default to true which causes the attachments corruption with Gmail's IMAP

Current fix (as of Feb 2012) is as mentioned above to set the new variable to false.