The 6.0.1 Built-in Service Guide gives smtp as the example and does not explicitly state that http(s) and ftp are supported also, but they are.
webMethods Built-In Services Reference Guide Version 6.0.1
The value of the protocol key in transport indicates which protocol was used to invoked the service. For
example, if the service were invoked via the email protocol, protocol would be is set to email. transport will also contain a document (whose key is protocol-dependent) that holds protocol-specific details.
#webMethods#Integration-Server-and-ESB#webMethods-General