Attachments will be something like "Part1.2" on Thunderbird with Gmail IMAP

You probably have seen that attachment files of a message became a strange name such as "Part1.2". It seems that it will happen when you open a message with two or more attachments whose name include non-ascii characters and the problem stems from the way of fetching a message.

If you troubles with the matter, set mail.server.serverXXX.mime_parts_on_demand to false, where serverXXX is a server key of a Gmail IMAP server. In case you don't know the server key, type server in the Config Editor's filter. You will find the mail.server.serverXXX.name whose value is an account name of Gmail IMAP. Use the serverXXX for the mime_parts_on_demand option.

Please note that once you set this option to false, entire message including attachments is fetch when you open a message. Therefore, this workaround may lower the performance of your Thunderbird.

For information of this option, refer to MozillaZine knowledge base.