Several people who are using Gmail's fairly new IMAP feature have reported problems opening attachments through Thunderbird. Word and PDF documents, and possibly others, generate an error saying that they are "unable to open". Apparently this has something to do with Gmail reporting an incorrect size for the attachment. The same attachments open fine when using the Gmail Web interface instead of Thunderbird.
Here is the fix for this …
- Tools > Options… > Advanced > General > Config Options
- Search for "chunk"
- You should see a preference named "mail.server.default.fetch_by_chunks". Toggle it to false.
- Close and restart.
I found this solution buried in Google Groups.