Bugfixes for handling bad MIME type declarations; UTF16 characters in body

While importing several thousand real-world messages, I found a couple of bugs. I have created test cases for these bugs, which can be found in the test folder.

The first bug is triggered when an email has an attachment with the MIME type "image/jpg" instead of the proper "image/jpeg".

The second is triggered when there are UTF16 characters in the message body.