FIX: fix email replies for ms outlook

HTML from MS Outlook client contains too much information, so it's better to parse text rather than HTML in case of email reply from MS Outlook.

I am using `X-MS-Has-Attach` header to detect if the incoming email is from MS Outlook client, as that is [exclusive to Outlook](https://www.mail-archive.com/exchange@intm-dl.sparklist.com/msg10055.html). (not sure if it's the best approach)

[Bug report on Meta](https://meta.discourse.org/t/email-replies-from-outlook-not-stripping-out-original-message/21075?u=techapj)