Ram,
What is happening when you send something now (without defining the content-type in Notes)?
I believe IS should determine the appropriate content handler for the document based on the attachment extension.
You can see what IS is seeing by including pub.flow:getTransportInfo in your receiving service. This will show you the protocol (SMTP) and inside of the email document will reflect the contenttype it is receiving.
Hope this helps.
#webMethods#webmethods-Protocol-and-Transport#Integration-Server-and-ESB