Using Blat 3.0.7 on Win7 x64.
If I send an email attaching an .eml file (-attach option) I can't open the attachment from TB (this attachment looks empty).
I fount the problem was that TB cannot understand the "Content-Transfer-Encoding: BASE64" because it is upper-case.
Changing it to lower case solves the problem and lower case should be the standard (even if it shouldn't be case sensitive).
There is a post about it on moz forums:
Could you please change it to lower case?