sogo-mail: Priorize filename attribute to get attachment name

Some clients (such apple mail) use only the filename
attribute in the content/disposition header and the
name attribute from the content/type header is filled with
uninternationalized characters.

Example:

Content-Type: application/msword;
x-apple-part-url=C4977556-0D01-4C6C-8A51-451E0AADE431;
name=___.doc
Content-Disposition: attachment;
filename*=utf-8''%D0%A0%D0%95%D0%9A%D0%92%D0%98%D0%97%D0%98.doc

This changeset gives priority to the filename attribute.