Make mhstore(1) use "filename" attr of Content-Disposition

Make mhstore(1) use "filename" attr of Content-Disposition

Before, only the "name" attr of Content-Type was used as the filename
to store attachments. The "filename" attr of Content-Disposition
(which is the much more appropriate value) was ignored, thus often
resulting in the attachment being stored with a default name like
20.2.pdf, for instance.