This document explains a work around for downloading attachments in Thunderbird.
This problem generally occurs when trying to download large attachments in Thunderbird. By default, Thunderbird downloads attachments in chunks and sometimes it doesn't download all of the chunks. This document explains how to force it to download attachments in one large chunk.
Open Thunderbird and go to Tools > Options.
Go to the Advanced tab > General > Config Editor....
mail.imap.fetch_by_chunks in the Filter field. Right click on the item that comes up and click on Toggle.
mail.server.default.fetch_by_chunks in the Filter field. Right click on the item that comes up and click on Toggle.