After working my email hosting provider, they found a solution to a recurring problem I have with getting full email attachments.  In my setup, I use an IMAP connection on Thunderbird (version (20090605) ) with my email hosting provider.  And sometimes, I cannot pull down full attachments, especially when the attachments are standard ASCII text files.

After working with tech support, they requested that I make two (2) configuration changes, which after restarting Thunderbird, cleared up the problem.

In Thunderbird, navigate to Tools > Options > Advanced >
Config Editor.  Look for the "mail.imap.fetch_by_chunks" and "
mail.server.default.fetch_by_chunks" values and set them to "False".
Restart Thunderbird.