Hi Rams!
First, I'm not sure you are able to fulfill your requirement even with new functionality of mail adapter since it is intended for XML attachments processing:
Attachment.ContentType : This parameter is mandatory when the above parameter(Mail.GenerateXIMsg) is configured to 'true'. This will help to process only those attachments whose content-type is text/xml. Hence the value of this parameter must be text/xml.
If Attachment.ContentType = application/xml and the mail message contains 3 XML files(.xml) and 2 Text files(.txt) then 4 XI message(1 for payload and 3 for XML attachments) will be created and sent to IS. Hence we will ignoring Text files(.txt) files.
I would recommend you to use AttachmentSplitterBean developed by Eng Swee Yeoh.
Regards, Evgeniy.