Hi!
I use roundcube 1.0.2, and got a problem.
If I send email with .mht (https://en.wikipedia.org/wiki/MHTML) file attached rouncube web mail can't open it. While regular mail client like Thunderbird has no problem with it.
I think it depends on the mht content, because 0 bytes mht opens normally.
I've found one header that ruins everything.
My mht contains header From: <text in russian> like
From: <Сохранено Windows Internet Explorer 7>
at fisrt line and it's the root of problems. Removing it solves the problem.
Could you provide a sample message source? You could also open a ticket on http://trac.roundcube.net