Backport fix from php-mime-mail-parser to fix attachment parsing

Summary:

• Allow proper parsing of attachments with missing Content-Disposition header

Test Plan:

• Create application email for Maniphest.
• Send example broken email from Outlook 2007 to that address
  Outlook_Inline.eml38 KBDownload

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D16584