Hi Gopal,
PayloadSwapBean just replace the attachment of the email to a XI payload message. So by default the attachment needs to be a XML for further processing.
You can try to use MessageTransformBean after PayloadSwapBean in Mail Communication Channel - Module and try to convert the text to xml.
If you dont achieve with this, then you need to go for either Custom Adapter Module development or Java Mapping.
Regards,
Nabendu.