I use Blat 3.07 to send a .eml file as attachment (-attach option) from Windows XP.
Resulting email has an attachment like this:
trying to open that attachment from ThunderBird 16.0.2 gives me an error (attach looks empty).
I found that editing the content type to application/octet-stream solves the problem.
rfc2046 states (page 28, par 5.2.1)
No encoding other than "7bit", "8bit", or "binary" is permitted for the body of a "message/rfc822" entity
.eml files should be marked as message/rfc822 (as far as i know).
maybe Blat should send it using the 7bit encoding option instead of base64 if the content type is rfc822.
Outlook can open both emails with no problems, but it looks like Blat is composing the email in a non-standard way