Support RFC 2231 data format

[jkbzh]

Hypermail fails to find and save the attachment filename when a mail client is using newer MIME attachment standards than hypermail recognizes. Newer Mail clients may split the filename into several fields following the procedure in RFC 2231. hypermail does not recognize this data format, so it failed to find the filename, generated its own, and as a consequence the browser doesn't know to display the file as a PDF. In my personal opinion, we should teach Hypermail to read filenames and other attachment data following RFC 2231.

[vivienlacourba]

FYI I believe that roundcube pushed a similar changes in their latest 1.5.0 release.
roundcube/roundcubemail#6783