Control whatever to read attached RFC822 mail content

Reading into the attached RFC822 message as osTicket does by default may cause troubles when the first mail section does not contain a content type text/html. OsTicket will in this case use the body from the attached mail as content as it searches recursive for text/html content first. Email clients will normally show this messages as attachments

We have several cases where emails are handled wrongly especially spam reports.

This change will add an option in the admin panel settings->email tab to control whatever the inline message content should be read or treated as an attachment by osTicket
Furthermore during my test I have discovered that not all attachments shows up in osticket. I think this is due to a invalid filename (taken from the subject line). To fix this I have changed:added the additional line in the `$subject = preg_replace("/[^a-zA-Z0-9\\s\\-_\\.]/",'', $subject);` and truncated the filename after 40 chars.
After this changes I haven't got any mails where the attachment was missing.