It seems that 188.8.131.52 is not handling the following MIME type correctly. (on XP Sp2)
Content-Disposition: attachment; filename="filename.whatever"
I've got some PHP scripts that automatically build files and email them to me as an attachment using a PHP class called htmlmimemail. Generally these are just ASCII text files... but some may not be... For my purposes I don't care if Thunderbird KNOWS the associated MIME type for a given file.... In the case above, Thunderbird should see the data as an attachment... and allow me to save the file as 'filename.whatever'. What's happening is Thunderbird is displaying the data inline - without decoding the base64 - as garbage.
As I understand it, this is kinda what the 'application/x-unknown-content-type' MIME type is for.