mail: get_attachments returns correct filename when it is specified in content_type

[romsahel]

Changes proposed in this pull request

Found in production, an e-mail with the following part:

Content-Type: application/octet-stream; name="document.pdf"
Content-Transfer-Encoding: base64
Content-Disposition: attachment

Currently, this returns filename "Unknown" since there is no filename property in :content_disposition.
This issue handles the case where the name is specified in :content_type instead.

[andrewtimberlake]

Thank you, added in a6b537b