Added 'XML' to the array of static types for attachments. If an att…

[akopciuch]

…achment was on an email of content-type text/xml as opposed to application/octet-stream, it was being left out of the attachments of the message

Is the pull request based off the lastest version?

yes

What features have you added?

none

What bugs did you fix?

Added 'XML' as an attachment type so text/xml attachments are included in attachments

Is your code valid PSR-2?

yes

Have you properly documented your code?

yes

Has anything in your pull request already been fixed?

no

Anything else?

no

Optional things below

[] Added your name to the credits