txt attachments have incorrect boundary written

Prior to this fix, if a MIME email contained an attachment that had a content-type of text/plain, Crossware Mail Signature would incorrectly detect that as a "Plain Text" email component and append the plain text signature to the email.

This would also occur for html attachments, the html signature would be appended to the file.

This fix correctly detects when the mime part is an attachment in the email.

This fix was implemented in V4.02.