its happening because you are not specifying the content type when sending email please refer to the following documentation
7-1-1_Integration_Server_Built-In_Services_Reference
you will find it @ the following hierarchy
<your_directory>:\webMethods7_documentation\Developer\7-1-1_Integration_Server_Built-In_Services_Reference.pdf
If you are using filename to attach a file to the message and the file is not a plain text file, you must set the contenttype and encoding. For example, to attach
contenttype: application/msword
filename: mydir/myfile.doc
encoding: base64
#Integration-Server-and-ESB#webMethods