Fixed issue with mime type

Finally I used **davidpersson/mm** library which requires PHP 5.5+. This is the most discutable part of this PR. PHP 5.5 was released a year ago and has several minor versions released after that so I consider that we can safely migrate to PHP 5.5. 

Is somebody against this change?

I would release 1.2.0 API when this fix is merged becase it changes the way to work with attachments.