I noticed sup has trouble handling attachments sent by Roundcube webmail. 
Somehow, those mails use an alternative encoding of filenames, specified in 
RFC2184:

    Content-Transfer-Encoding: base64
    Content-Type: image/jpeg;
     name*="UTF-8''20090912-i004232-gr000.jpg";
    Content-Disposition: attachment;
     filename*="UTF-8''20090912-i004232-gr000.jpg";

Sup fails to detect these filenames.

When trying to save these attachements, the filenames generated by sup have a 
trailing semicolon.

The attached patch is a quick and dirty fix for these specific problems.
There's probably a better way to implement this.