*.msg attachments sent from Outlook are not detected as attachments

I've encountered issue when processing email which was sent from Outlook 2007 and has *.msg attachment in it.

You can see full raw message in my commit, here is a part of it:
```
------=_NextPart_000_0EE8_01D00D91.AB93BEF0
Content-Type: message/rfc822
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment

...
```

Attachments that doesn't contain filename in the Content-Disposition header are skipped. However any other email client (Gmail, Mac OS Mail, Outlook) processes them correctly and just assigns some random name to them.